As others have noted, great question!  My unequivocal answer - NO!  But...  We should each make our own choices based upon the dynamics of the moment - there are weekends where I definately want/need to be "disconnected."  I'm not referring here necessarily to an Internet connection - with Blackberry's et al, there's hardly ever a reason to not be connected if you so desire.  I'm referring to those weekends where you either mentally need to disconnect, or where you have family/friend commitments that deserve your full attention.  I want to feel free of any conscience with regard to not being responsive.  The other side is the nature of the request - if the need is sincere, could not have been better anticipated, and has real urgency to it (as opposed to the fake urgency some people attach to every request), then, within the limits of the above-stated need to disconnect, I will happily "go the extra mile" to help out a colleage.  But that should not lead to an expectation that being available each and every weekend is the norm. 

On the other hand, there are many weekends where I don't need to "disconnect."  In those cases, I'm happy to respond, engage, and do whatever is need, or that I want to do.  I do that not because I feel I have to, but because I want to.  I enjoy (most of) the work, I enjoy responding to and helping a well meaning colleague who makes a well-articulated and reasonable request.  I believe that what goes around, comes around.  If I am generally responsive to people, they will be responsive back.  If I allow myself to be abused by disorganized people, who wait till the last minute to make requests, always need the response "in the next hour", and are so inarticulate about the request that I have to waste time trying to decode it, then I'm being taken for a sucker.  As a mentor taught me many years ago - don't collude with dysfunctional behavior!

I'd also point out that your "weekend" question is really just a special instance of "how responsive do I need to be."  Again, what goes around, comes around.  Treat others as you would be treated.  Do what you say you will do.  I think these principles work pretty well.