I have 3 main subsites to deal with Big Ben's Victoria, Terragen and Panoramas. The last two are fairly similar in nature... technical tips, resources and showing off, but the first one is substantially different and is going to take a lot of work regardless of how I approach it. At the moment the only problem I'm having with WordPress is getting the type of menus I want (or rethinking the navigation entirely). I was contemplating having everything in a single system, but I think that would be overly complicating things from a development/management point of view.
I'll be starting with my Terragen site anyway as it has the most up to date content (in a state of semi-development) so it will be the easiest to mould to a new system.