Apple have silently changed the details of AirPrint support since the release of iOS 4.2 Beta 2 last week. Originally the AirPrint capability was listed by Apple as supporting the iPod touch 2nd Generation when it was announced back on the 15th September:
AirPrint will work with iPad, iPhone 4, iPhone 3GS and iPod touch (second generation and later).
After the release of the latest iOS Beta the press release has now been changed to read:
AirPrint will work with iPad, iPhone 4, iPhone 3GS and iPod touch (third generation and later).
So it looks like if you have an iPod touch 2nd Generation, printing will be out of reach for you. A bit sneaky to change a press release a few weeks later.