Until maybe an hour ago, my blog was still running WordPress 2.9.1. The main reason why is because my blog runs the WP Subdomains plugin, which stopped updating completely a year and a half ago while its author was in the middle of trying to update it for 3.0 (continuing a long tradition, as this plugin has gone through at least three different iterations with different developers). The only reason why I finally did upgrade is because I didn't realize the Subdomains plugin wasn't a revival of WP-Subdomains until it was too late (it converts all top-level categories to subdomains instead of only selected ones, and I can't apply different themes to each subdomain), but even before that I was thinking about possibly using WordPress 3.x's multisite features to get around the lack of updates for WP-Subdomains.
Although I know next to nothing about how to do this and so am not sure if everything I'm about to say is entirely true, the main downside to this approach relates to what most appealed to me about WP-Subdomains in the first place, which is that it allows me to maintain the sense that each category I split into a subdomain is simultaneously its own subsite and a part of the larger blog.
I'll try to illustrate what I mean. On my site, I have the main site, which I'll call foobar.com, and on that site I have two categories that I've made into subdomains, foo.foobar.com and bar.foobar.com. All my other categories are not subdomains and remain part of the main foobar.com site. All my posts that are categorized "foo" or "bar" are automatically linked to foo.foobar.com or bar.foobar.com from foobar.com, and appear on foobar.com like any other post on the main blog. There is only one post, on (say) foo.foobar.com, not one on foobar.com and another on foo.foobar.com. What's more, if I want to I can create a post that belongs to both the "foo" and "bar" categories, though I don't know if I've ever done so or how the plugin would handle it if I did, although I have created posts that belong to either the "foo" or "bar" categories and also other categories I have not made into subdomains, and do so all the time.
If, as I imagine would be the case, I program two subsites to rebroadcast the "foo" and "bar" categories on my main blog, I would keep the latter but not the former. On the plus side, I could have different widgets for each subsite, something WP-Subdomains doesn't seem to let me do.
Is there anything out there that would better approximate the effect I'm looking for, and can you give me any pointers on how to do whatever it is I end up doing? (Ideally, of course, someone would take over the WP-Subdomains plugin for at least its fourth iteration, but I don't have nearly that kind of time to do it myself... and for the record, I do have plans that would likely involve me using multisite anyway.)