With a new job, that want may actually be satisfied at some point in the near future.
Part of the reason I want to purchase a Powerbook is work related. We all use OSX (10.3.4 Server) at work, and being able to have hardware that can run the same software as at work is useful.
In addition to that, the machine I have at work is really slow. 16mb video, 384 RAM, and a 450MHz G4. I don't know a lot about Macs in general, but I do know that the Powerbook I want is a 1.33 GHz G4. Even just basing it off Mhz seems to indicate that there might be a slight speed difference there.
So, for work purposes, I'd be able to get used to the operating system I use, and have a faster machine. At work, I have a dual input monitor - and I wouldn't be surprised if we didn't have the connectors to make the laptop go to the monitor. Even if we didn't, I bet I could get it paid for by the company as a work expense. I'm seeing a serious lack in productivity compared to my normal due to the fact that the computer is significantly slower.
Speaking from a more personal point of view, the Powerbook has a lot of toys I want to play with. The bluetooth support in OSX is way better than anything the PC has to offer (at least, that I've seen). Since my phone has bluetooth - indeed, it's the best way to interact with the phone - this is a nifty concept to me.
I've also wanted, for a long time, to have a computer that I can take with me almost anywhere. Being as small as it is, the 12" Powerbook fits this role pretty well. Add the unlimited internet I get with my cell plan, and I can get internet -at least to IRC with - just about anywhere. Quite nice for weekend getaways. ;)
I want a small computer which I can take anywhere so I can take it with me when I go other places. I like having a desktop most of the time, but occasionally I want something a bit more mobile. Between my personal wants and the ability to use it for work - plus, the whole idea of having a completely different architecture appeals to my geek sensibility.
I plan on buying a Powerbook, and I want it as soon as possible. Luckily, that's looking like it could be a reality in the near future.