Application stuck in "Installing..." stage in Market
I just had this problem, and somehow I got this solved.

What I did:


Entered Settings -> Applications -> Manage Applications.
Selected the Market app, and "uninstall updates".


After that, I entered the Market app again, and I was able to reinstall the app who got stucked.

PS: I'm not sure if this was just a coincidence, or this procedure works always, but it worked for me.
PS2: My Android version is actually in portuguese language, so I'm not sure the option names I referred to above are correct in english.