Thanks for the quick replies.
There is no particular reason for 32bit PC's apart form we just haven't started the migration to 64bit yet and were wanting to start testing for Engineering & Computing software. (There may be some historical financial software that may prevent moving some admin pc's but I expect all student PC's to complete the migration at some point.)
My problem is that we have over 1700 PC's and Laptops and there is no way these can all be updated at the same time, or across a few weeks, so we will definitely have to co-exist during the migration. (We have over 100 different software packages and applications across multiple sites so even using SCCM it would be a very tall order to expect migration to happen quickly).
MS definitely say that sharing the roaming profile between the 2 OS's is not supported but some people have blogged saying they haven't had any particular problems. I'm preparing for worst case and there wanting feedback on what issues people have found and any suggestions?