I'm running most of my Hyper-V lab Server 2008 instances on 512-1024MB ram and haven't had many performance issues on them. The x64 versions do tend to respond slower to management but I'm thinking this has more with the VT technology to do than memory bottlenecks.
Certain services will get in the way and kill performance and one of those is SQL server - that includes MSDE/SQL Express instances that are barely used, for some reason. Like a default install of Forefront TMG, or DPM or anything with its own local SQL Server instance will feel slow to manage. Removing the local instance and using a remote SQL however and the server will become zippy to manage again. The actual service performance however doesn't really seem to be slow in either case, it's just a pain to manage.
Infrastructure Server Core instances with AD, DNS, DHPC and DFS seems to zip along just fine at 512MB. As long as the AD database isn't big enough to not fit in memory there shouldn't be much problem ^^
I've also noticed that IIS7+ enjoys a bit more memory as well - but I run a few non-loaded ASP.NET & flash HD video streaming websites just fine on 1024MB x64 without any delays or page nor http file transfer performance troubles...
...for real production use though, best follow the suggested minimum which certainly is 2GB or more, depending on applications.