There are many pages out there about how to use a tethered BlackBerry internet connection with Ubuntu 9.10. Here is one that actually works. It uses Barry, BlackBerry support software generously provided by Net Direct Inc.
I’ve found this quite useful with an Ubuntu based netbook. There is Wifi a lot of places, but not even close to “everywhere”.
My BlackBerry is on the T-Mobile network, which (nicely) includes tethering at no extra cost, but (not so nicely) offers only EDGE (not 3G) in most of the US. Still, in a pinch an EDGE connection is far better than no connection, and is quite suitable for occasional use at zero incremental cost. For heavy mobile wireless tethering users, I suggest Verizon or Sprint service with their respective USB dongles.