I'm actually posting this a day early, so if you want to print from iOS 4.2 and don't have one of those limited HP eprint machines, you can still print from your iPhone/iPad/iTouch
How to Install AirPrint to Enable Wireless Printing on Windows? | Jaxov
I followed the instructions and they work fine. I need to add that you need to run cmd.exe as administrator and the firewall dialog may not pop up automatically. You can always create your own firewall rule for Airprint.exe. That's what I did. Please note that on a printer that will do full duplex, it defaults to printing on both sides, but that can be changed. You all need to do is enter your Windows username and password, and you only have to do that once on each iDevice. Other than saying that my "toner" rather than "ink" was low (a very small deal indeed,it worked perfectly. This is an excellent, free solution to Apple's dropping AirPrint from this release (presumably due to a patent troll).
The other really nice gift from Apple is the free release of "Find my iPhone" for the lastest iDevices.
Week in Apple: iOS 4.2 finally lands, we all find our iPhones
It is important to note that if you have one qualifying iDevice, set up the free limited MobileMe on that device (like my iPad), and it can then be set up on any iDevice you own that will run iOS 4.2, like my iPhone 3Gs. There is a Web interface and also free apps for this service. You need your iTunes/Apple username and password to make this work.
Apple Mobile Me Find My iPhone/iPad
Find my iPhone Web Interface