Is it possible to cut out part of a wall?
You can use the clip tool responsibly, I love to argue with msleeper.6:15 PM - >f7> msleeper: Yes I know.
6:15 PM - >f7> msleeper: But it's easier to tell people not to use it, then train them in the very specific way to use it.
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I really tried my darndest to get the clip tool to do what I wanted but I never got the cut I was looking for. It always wound up erasing my whole wall. Not what I was going for. Haha. I read some wiki's on it - the one artile that msleeper linked in the "everyone should read this if your new to mapping thread" was helpful in general but I still don't get clipping. If you can actually for instance cut a doorway using the clip tool - I'd love to know.
You can use the clip tool responsibly, I love to argue with msleeper.6:15 PM - >f7> msleeper: Yes I know.
6:15 PM - >f7> msleeper: But it's easier to tell people not to use it, then train them in the very specific way to use it.
I do not love to argue with you! You made that up! 
(it's funny because I'm arguing)
The clip tool is easy to use once you realise that it has multiple modes. Try the following:
1) Select your wall
2) Click the clip tool
3) Draw a vertical line where you want your slice to be (don't press enter yet)
4) Click the clip tool button twice more, to cycle through the discard modes.
5) Press enter (Ensure both sides of the cut are white, not red, which means everything will be kept)
6) Deselect the brush, which should now be 2 seperate pieces
For a doorway, just make a second vertical cut as above, so your wall is in 3 pieces, then adjust the size of the middle piece to form a opening. For a window opening, make a horizontal cut through the middle piece, then adjust as required.
It sounds like a lot of steps, but is actually incredibly simple in practise. When making architecture, I usually start by laying a single brush, then just extend and clip a bit off to make my next brush, and so on, rather that repeatedly making individual brushes with the brush tool.
Or, simply, pre-plan everything, and never make mistakes.
*(i'm only trolling, because anything i could have said has already been covered, from the "use the carve tool" to the "use it only sparingly", to the "ungroup, remove one wall, and rebuild it the right way.")