Here you can see the schedule of the semi-finals: Fixtures, Schedule | ICC World Twenty20 2009 | Cricinfo.com
In Nottingham it's E1 (most likely South Africa) v F2
In the Oval it's F1 (most likely Pakistan) v E2
In other words, Pakistan will probably play in the Oval in London.
I hope thats thats not true because I only match I watched was the warn up game in Nottingham..which sucked because they lost