Sure that would work fine, but the only problem would be having to keep two separate subscriptions going: one for your data via the modem, and a completely different subscription for the Blackberry.
Why not forget the Blackberry, when any decent cellphone these days will also access your email (using the same SIM card as your data modem, which keeps things simple, and cheaper), the only difference being that doing it this way means you have to 'pull' the email on your phone where a Blackberry 'pushes' it - - - meaning that you physically need to check your email on your phone as often in a day as you feel necessary, rather than waiting for your Blackberry to beep. Not a great drawback, surely?
If you have the option of choosing a provider, you might also like to look at the coverage maps to see which has the best EDGE where you are going -- that's if you are going to be away a lot from the urban 3G/HSDPA signal; GPRS is quite painful on your laptop, it can be so slow, and EDGE is a definite improvement (theoretically about triple the speed of GPRS, and more stable). The consensus of this forum seems to be that MTN on average has better EDGE coverage out in the rural areas, while Voda is strong with its very high-speed transmissions in cities and (some) towns.
Don't bother with contracts unless you really have to, by the way. Not only do you get locked in, but prepaid data is actually cheaper in many instances, certainly when you buy airtime through discounters like Pick&Pay. No connection charges either -- just a couple of rand for a prepaid SIM!
