The problem with doing a web browser is that it's almost impossible to do one with 100% of the functioanality you get on a computer web browser, due to the various plug-ins that PC and Mac web browsers end up using to extend their functionality. If you've ever tried a web browser on a non-computer device -- a Pocket PC, PSP, PS3, Wii, etc. -- you know it can cool, but can be frustrating when you hit a purchasing page, video page, etc that doesn't work. That said, Microsoft has a lot of experience with building good web browsers, so I'm sure something cool could be developed.
No idea if a web browser will ever come to the 360, but I do know that the folks who'd make that decision have heard the requests! It's all a matter of resources to some degree. Do you want a web browser, or more gaming-related features?
As for Castlevania... It'll be here next Wednesday!