Here is a list of solutions many people came up with, and why they won't work. After that, I will tell you the solutions that will work.
1) bar-codes or QR codes. These are just images. They can be copied. Obviously scanning at entrances causes insane traffic. Even if you scan, the fake badges will have valid codes since they are copied from a real badge. This only works at baseball stadiums and such where there is no re-entry and scanning everyone at the entrance is reasonable. There should be a unique QR code on every badge, but that should be used for games and such. It won't help detect forgeries.
2) UV ink or tiny holograms are surprisingly available. I saw many forged badges, and they were quite sophisticated. Forgers will get UV ink. They will print tiny hologram stickers. These will not help.
3) Not mailing badges is the worst idea in the universe. Some conventions don't mail badges in advance. That reduces forgery greatly, but not entirely. There are people who can print badges overnight. Also, it brings great suffering to all attendees. It means we have to wait in line to get badges, and that means a lot less time for actual PAX. Even if we are allowed to pick up badges early, that effectively increases the cost of PAX because you have to pay for extra hotel nights and days off of work. Even if forgery runs rampant, PAX should always mail the badges because the benefits of not having that registration line far outweigh the cost of forgeries.
Here's the real answers that will work.
Make the PAX badge insanely awesome and electronic. Take for example the badges from DEFCON. Make the badge include a very small USB stick, maybe 32MB, that contains PDFs of the program, schedules, maps, etc. Also include an RFID, make it a full hologram on one side, with a QR code (for fun and games, not for verification), and maybe even a programmable LED display. Also possibly include wireless, a tiny sound emitting beeper, or a vibrator.
The badge will not only be insanely difficult and expensive to forge, but it will provide a great deal of entertainment and interactivity for the people of PAX. For example, you could just have your badge scanned to enter into a tournament. Tabletop library could scan your badge when you check games out to save time (they would still take your ID). Exhibitors could scan badges and have people wait in line virtually instead of waiting in a physical line. They could setup a big screen to show who is next in line. PAX XP could integrate with the badge. Community groups like Pokemon League could use the badge as well. If the badge included wireless and vibration and/or sound, you could even use it like those things at restaurants that tell you when your table is ready. Or maybe have treasures hidden around the convention that cause nearby badges to vibrate, so the badge acts as a treasure detector. There are a zillion possibilities.
Now you are going to say that this badge will be very expensive. Even with a bulk discount, the badge will probably cost $30 each alone. That's more than half the cost of PAX to begin with. How do we get this money? One, you could raise the cost of PAX. I actually don't think $75 is unreasonable. The economy is bad. Money is worth less. If they don't raise the price, then PAX actually becomes cheaper each year. Another idea is to raise the price of badges over time. People who register immediately pay $50, but people who take their time and register at the last minute pay more. That is standard practice at many conventions to encourage people to register as early as possible.
But there is a better way to pay for these badges that does not require PAX to raise its price. Have advertisers pay for it. Have vendors pay to have their ads included on the USB stick in the badge itself. It's definitely possible to find a handful of companies that will pay a few dollars per badge to have their demo or advertisement included on it.
Also, PAX should have a legit official badge exchange. At least endorse an official one, like StubHub or something. Or maybe even let people return badges for a refund and then resell them. Then put out the word on craigslist and everywhere else with clear instructions on how to make sure a badge is legit. And let people know if they buy a badge from any unofficial source, they must verify it is real before paying, otherwise they will be kicked out.
One last thing. Obviously checking badges at the doors of the convention creates an insane traffic problem that just isn't going to work out. However, there are many places at the convention where lines already exist. While people are sitting around waiting, it's easy enough for an enforcer to walk up and down the line checking badges of these people who are just sitting around. Main theater line, queue room, lines for games in expo hall, lines outside panels are all good spots to do spontaneous casual badge checks. While people are sitting there waiting, have someone go up and down politely asking to see every badge is not unreasonable. You can also check when anyone signs up for a tournament or checks out a game.
You don't need to check 100% of the badges, just as many as is reasonable. This won't catch every forged badge, but it will go a long way. Also, it will make anyone with a fake badge scared to participate in anything popular. They will be mostly limited to walking around, looking around, and only participating in things that don't have lines or sign-ups. People who know they have a forged badge will walk around the Expo hall afraid to actually participate in anything, and they will definitely not get the same experience as a legitimate PAX attendee.