Here are some more gotchas. Some I have run into and some just came up when I searched for "Builtin Administrators" in the MS Knowledge Base with SQL Server 200 selected as the product.
PRB: SQL Server Full-Text Search Does Not Populate Catalogs
BUG: IsAlive check does not run under the context of the BUILTIN\Administrators account in SQL Server 2000 Enterprise Edition
Be especially careful on clusters. See the following article for more information on this topic in relation to clustered instances of SQL Server:
INF: How to impede Windows NT administrators from administering a clustered instance of SQL Server
There were some other articles that came up in the search but I did not include the ones marked "FIX" or "INF" except for the one clustering article. Basically, make sure everything is working right before you remove this group account. That way, if anything breaks, you know exactly why. Then you can put the group back in and research the proper solution.