I really do not know what Apple have done this. I had a MB 2007 and XP used to install fine. Then the superdrive on that went and you cannot install another OS apart from OSX via an external DVD drive (I do not have a superdrive, but apparently that would work).
I have a 2011 MBP now and I thought the installation of W7 would have been easy and simple. No such thing. Why are Apple not fixing it?
I think Guido's fix only applies to MacBook Airs.
I found this article (before I had a W7 install disc) which may help some people. I was not able to succeed with this as I tried it with XP.
http://insidethebrackets.blogspot.co.uk/2009/04/install-windows-on-macbook-air-w ith-no.html
How I got it to work on my MBP? I had to install XP. I went through boot camp until the MBP went to restart, then added the XP disc and installed it. NOTE, if you want to subsequently upgrade to W7, then ensure the XP partition is formatted as NTFS, otherwise you will not be able to upgrade to W7.
Spent 12 hours of my Sunday on this today, hope to upgrade to W7 tomorrow. Will update this thread if I am successful.