The purpose of caucuses is to provide a gathering point and support for minorities within a larger group whose views and interests might otherwise be diluted or ignored by the larger group. I'm not talking just race: you could have a female caucus, an opera fan caucus, what have you.
Caucuses, being informal groups, should have the right to include or exclude anyone they want.
A white caucus would be pointless, because the entire Congress is a white caucus. It misunderstands the point of a caucus. That said, Tancredo has every right to form one; he would just be displaying ignorance.
Here's a list of Congressional caucuses. Note that the only common principle is that they represent smaller interest groups within the larger body of Congresscritters.
http://en.wikipedia.org/wiki/Caucuse...tates_Congress
So race-based caucuses are fine, in my book. Not all caucuses, of course, are equally deserving of respect.