I'm in the get-two-Macs camp. There are a few reasons for this.
First, as much as we don't like to think about it, computers occasionally have problems and even die. If you have two Macs, synched with .Mac and backup key data from one to the other, you're always going to have a Mac to use, and if one gets sick you won't end up putting off bringing it in because it's your only Mac. As a result, both Macs will tend to stay healthier because you'll bring them in when they need it. No "dear God, can I live without my Mac for a week? Maybe I should wait..." issues.
Second, as much as I love my laptops (I have a 12" PB and, through work, a 15" MBP), having a fixed system is nice - external drives are always there, you can use the web server, you'll have more power and/or disk space, etc. And, as much as I love my fixed systems (currently just have the 17" Intellimac and mini duo connected), often you'll want to surf on the couch or when traveling. So both have their strengths and weaknesses. By getting one of each, you have access to the strengths of both.
Third, using the built-in VNC server and Chicken of the VNC, you can always access one Mac from the other, meaning you can be burning a DVD on the iMac while, at the same time, surfing MR on the MB and checking on the iMac from time to time.
Fourth, the 17" MBP is, as others have said, just too damned big to use on a plane. And you're going to feel sick as soon as some airport security goon scratches it, whereas an MB can be fixed with various compounds.
But that's all just my opinion. I personally get a lot more use out of having both types of system around than I would having just the one type, and I don't feel that, for you, the 17" MBP is worth an entire iMac more than the MB.