That was a really easy thing to do with Win2k / IIS5, but you can run into some weirdness.
If you're doing really basic files it works okay, maybe even with some light permissions, but if you get into much more than that, it will start giving you trouble on a DC.
Some examples I've run into with 2k3 server (not 100% regarding IIS)
Shared Fax flat doesn't work right on a DC, it'll seem like it's working, die after a reboot, or just stall out and look like it's running but it's not. It has to be reinstalled if it quits recognizing the device for some reason. On all the same hardware with a non-DC, everything works Peachy.
Software Update Services works great on Win2k/IIS5, and pretty well on a Win2k3/IIS6 DC, but it can do some weird things to IIS6 configs that make other sites you may have on there start behaving strangely with permissions and stuff.
Sharepoint works great as long as it's on a non-DC. The only big thing I found out with this is the way the security is implemented is a little funky in that an administrator on the local machine has access to all the sites, and you wouldn't expect it to work like that. On our old Intranet local admins could be permissioned out of stuff easily, but on Sharepoint it doesn't work quite like that.
Those are the three things that I recall off the top of my head that I was banging my head against the wall trying to figure out. None of them like being on a DC.
The big stem point for IIS is that IIS really likes to have a local account for anonymous access (the IUSER_COMPUTERNAME account), if it's on a DC, then there are no local accounts for this, only directory accounts. IIS5 could put up with this, but IIS6 really doesn't like it.
It's best to not run IIS6 on a DC unless you absolutely have to, and even then, keep in mind that it could very well act different than it did on Win2k.