This is exactly what I'm dealing with, too.

I keep wanting to learn Ruby/Rails (and I have been working my way through the Rails Tutorial book online, albeit with a break the last week or so for travel and stuff). At the same time, I keep thinking "You know, I have so much domain experience with C# and ASP.NET, so would it be a better use of my time to focus on learning ASP.NET MVC and improving my architectural skills?"

I'm still working through that, but in the mean time, I'm pushing myself to at least go through the Rails tutorial (and probably an all-day Rails session at the CodeMash precompiler day) on the grounds that it's probably a good thing for me to break out of my narrow MS tech focus and gain exposure to other ways of doing things.

I think the only real constant in this field is that if you're not continually learning something, you're falling behind.