Although the next municipal elections have not been finalised [they're either in April or May, but a specific date has not been set], registration is on 5-6 February 2011, ending at 5pm.
I tried the sms line [unregistered] but received a different answer online, so make sure.
Follow these links to find out where to go, what do and if you're registered:
Am I registered.
To find out WHERE you can vote, go here.
However, if you're outside your voting district on Election Day you may vote at another voting station in South Africa. If you're outside the province where you're registered, you'll only be able to vote in the national election and not the provincial election, and you'll be asked to complete a form (VEC 4) at the voting station.