Steven Harms has a fairly completer rundown of what you can and can't do with Ubuntu. It's a good list to refer to if you're undecided whether to Ubuntu or not.
In a nutshell, the Can'ts involve:
*Games written for Windows
*Specialized hardware
*Proprietary file formats (XLS, DOC, PPT)
*Proprietary multimedia playback
And the Cans involve:
*Creating and playing media in open formats
*Point and click install of software
*Creating documents
*Network connectivity via web, email, and instant messenger
*Printing
*LiveCD install