We have a SharePoint server farm that has both Windows 2003 and Windows 2008 servers as front end.
Has anyone else tried this configuration? Is it supported?
If they are both the same bit, 32 or 64 they should be ok. The only time I have really heard of this are when people are migrating, no long time operations.
ISS 6 and 7 are going to be the problems. Basic operations should work since the WFE will work with the correct APIs for modifing the web.config. You will probably run into problems with FBA since that has changed between versions, also keep track of what can and cannot be done on each WFE will be a pain.