I was thinking roughly the same , doing web clusters on Raspberry machines. Mostly because I don't need the performance just a bunch of machines to test stuff on. 10-15 Raspberry's will require just fraction of power compared to my HP DL360 that is acting as virtualization host now.
I compiled and ran Nginx without any problems last night on the default Debian install, going tho get php running tomorrow I hope. I will be running the database on my 'powerhouse' Via C7 based machine
I really don't see the need to strip the OS since I will only run what I need. Kernel is module based and will only attach needed modules so there is no point in running lightweight distros unless you have limited storage.