Problems with Windows 2003 & 2000 running in the same Domain!?

No problems... but be carefoul at domain functional level if the DC are mixed.

There are some issues, but they will most likely not affect your services or available functionality, but rather affect the way you configure administer the domain and its services. One good example is single-label domain names. By default Windows 2003 (as well as Windows 2000 SP4 though) will not attempt to register or update any dns records located in a single-label domain zone, e.g. "local". Effectively, this may result in incorrect service record registration for newly installed domain controllers with W2k3. Windows 2000 prior to SP4 will do, however. Windows 2003 and W2k SP4 will need some additional configuration to make them able update records in single-label domains.

Single-label domain name issue is the one I see most often; for more details see KB300684, Information About Configuring Windows 2000 for Domains With Single-Label DNS Names

To be more precise: Are there ANY problems known when Win 2000 Domain-Controllers and Win2003 Domain-Controllers are both present for the same domain? Is this recommended? Is this common practice?