I use a 13" MacBook Pro with a relatively small SSD and a boatload of RAM for my day to day development work, and I love it. I've got a Windows virtual machine for stuff like Remote Desktop or WinMerge, and I've got most of my home folder (including my working directories) mirrored between the host and guest OSs so I can quickly switch back and forth between the two environments. It works great. I keep both OSs pretty lean, so the whole thing runs VERY fast.
As far as screen space goes, I guess it really depends on what tools you use. I spend 90% of my time in gvim (or MacVim), which scales very well to both tiny and huge screens, so while I like having more space available, I don't feel cramped on a single 13" either. When I don't have an external, I pretty much run all my apps maximized and just command-tab between them. If you're using Visual Studio 2010 with all the toolboxes and junk, that might be a problem.
As someone who DOES keep up with all the Apple rumors, I see no reason why one should hold off on buying a 13" MacBook Pro. I'm not aware of anything interesting coming down pipe. Sure, spec bumps will be here sooner or later, but the 2.66 GHz Core 2 Duo is already lightning fast for development work.