Flash won't automatically pop up.

[Loelke]

XSI

[Skirball]

If you push the flash button (side of lens mount area) does the flash pop up?

If so, what shooting mode are you using?  Certain auto modes disable flash, as does full manual.

Also, check out the Flash Control settings under one of the menu options, just in case something got switched accidently.

[Loelke]

I'm in the"P" mode.

Yes, it will pop up with the button.

[Skirball]

If you're in P mode, in a dark room, it should pop up.  Did you check the flash control menu?