Hi there!
I know that some peoples still can't get it to work in Snow Leopard (10.6 and 10.6.1).
So let's try to collect everything you need in this post:
0. Turn off printer.
1. Download an CAPT Driver:
http://software.canon-europe.com/software/0028785.asp
2. Download this patch
http://www.elverils.com/downloads/LBP2900.gz
3. Install CAPT and reboot
4. Open LBP2900.gz and copy file in /Library » Printers » Canon » CAPT » Profile » Device
5. Turn on printer. And open Printing&Fax prefpane. Click on [+] to add new printer.
6. Select Canon LBP2900. If at this step in "Print Using" field is "CAPT Printer" than you're done. Print & enjoy. If not — go to step 7 or 8.
7. Something wrong, and now you have to try to reset printing system. You will lose already installed printers but still it's good 🙂.
7.1. Turn off printer. Close "Add Printer" window and in "Print & Fax" window press alt+right click on "Printers" list. You'll se menu with only one item "Reset printing system". Cross your fingers and click it.
7.2. Go to step 5.
8. If you done this steps, but printer still not installed — you can replace step 4 by downloading full CAPT directory and copy in Dexter_HD said before in this topic. After this you have to reset printing system as well.
Thanks and unlimited printing for you 🙂.