This *is* possible! Android has you covered!

If the N900/BB is acting as an AP (access point) via WiFi, then yes, the Tab can be tethered to it in that way. After all, it's just connecting to another WiFi hotspot -- the N900/BB being the hotspot in this case.

Also, with an app called pdanet, you can do USB and bluetooth tethering without command line hacking or root access. But it is costly, it's going for $16 right now.
http://www.junefabrics.com/android/

Very nice.