Thanks to kbalertz, I was notified of these:

FIX: You may receive results that are not correct when you perform mathematical computations that involve floating point arithmetic in Visual C++ .NET 2003

The repro code looks like this:

double a = 1.0 - sqrt(225.0) / 36.0;
double b = 1.0 + (31-30.0) /70.0;
if( b > 2.0 )
   b = 2.0;
double c = b * a * 9.0;
printf( "%f\n",c);

If all is well, this is supposed to print 5.325000 but if you have the problem it prints 14.378571 -- a slight discrepancy! Get the hotfix only if you need it.

Those with an old version of the XBox Developer Kit installed might also like to know about:

PRB: Debugger stops responding when you debug a Visual C++ .NET application with the Visual Studio .NET debugger

Workarounds: get a newer XDK, disable parts of the XDK, or leave your XBox on and connected to your computer all the time. That last one would kill my productivity for sure.