in case anyone using a Dell Mini 9 comes across this:

i was able to get wifi working on 10.04 without ever plugging into wired internet after installing from thumb drive.

1) install ubuntu 10.04 from thumb drive.
2) boot normally.
3) insert the thumb drive you just installed from.
4) open the file manager, go to the thumb drive, and navigate to pool/restricted/b/bcmwl, and double click on the .deb in there. it will tell you it needs 3 dependancies - make a mental note of which three.
5) navigate back to pool/main, find those deps and install them.
6) install the .deb in pool/restricted/b/bcmwl
7) reboot