I have some personal stuff on my work laptop, such as personal files and some basic personal apps like my RSS reader and Twitter client (I use Windows Live Sync to keep my files in sync across all of my PC's). But my work laptop is pretty fast so I deliberately try to avoid installing most non-work related apps on it so that a) the Windows image stays de-crappified and fast*, and b) it forces me to focus on work when I'm on my work laptop and not get distracted with other stuff.

My personal laptop is an Asus UL30A, so it has the advantage of being very thin (thinner than most netbooks) and a long battery life (12 hours, more than most netbooks). So I can easily fit both my work and personal laptops in the same bag. That said, there are only 3 reasons that I'll bring my personal laptop with me on a business trip:

1) If I'm bringing my dSLR and need some RAW processing utilities (which I don't have installed on my work laptop)

2) If I have a long flight and want the long battery life of my personal laptop for the plane

3) If I'm going to be at some sort of conference where I'll be walking around all day and I want the light weight and long battery life of the Asus

If none of those 3 apply, than I don't bring the Asus.

*On an unrelated note, my employer rolled out Windows 7 images last September and my build is still running just as fast now as when I built it almost a year ago. Keeping the garbage apps off your machine really adds to the longevity of your build.