Currently all those XP users can easily run Firefox or Chrome and are therefore not stuck to IE7, but I really wonder how long that will be possible. Will it be 1 or 2 years until Chrome won't run anymore on Windows XP? (Firefox has problems with the linker of older visual studio compilers already and any software compiled with a newer microsoft compiler will need the msvcrt update, unless you use http://kobyk.wordpress.com/2007/07/20/dynamically-linking-wi... )

Those XP machines will never be upgraded and many won't be replaced until there are no spare parts anymore.

One really can't trust Google to keep anything around for more than 6 months