Update - 12/24/07: This issue has been fixed in XNA Game Studio 2.0. Woo hoo!
While working on my game for Maker Faire (which you'll be able to download in about a month), I came across a timing bug on the Xbox 360 using XNA. For an application that requires very accurate timing (millisecond precision), one can use the StopWatch object in .NET 2.0. This was working quite well in my PC build, but on the Xbox 360 build, I would notice that time would drift with no explanation. After digging around for a couple days and writing a simple sample to prove the point, I determined that the time value returned by the StopWatch object would drift by about 150ms/minute, which is considerable when precision within a few milliseconds is required (and considerable when it should be accurate to begin with).
I eventually found a workaround: DateTime.UtcNow.Ticks is perfectly accurate on both platforms. So a quick rewrite to use this property in all locations requiring accurate timing fixed everything quite nicely.