I'm using the Claws-mail - it's been the most usable email client for the N900 for my IMAP accounts. However, it doesn't have a calendar. The vCalendar plugin is downloadable in source form, there isn't a plugin available from extras. Other plugins are available, so is there some reason why this isn't?

Is this a dead-end, and should I be looking at GPE calendar and how it works with Claws? Or can I compile for the N900?

I compiled this very plug-in myself using the maemo SDK.

http://maemovmware.garage.maemo.org/2nd_edition/

Can you elaborate... you installed the SDK environment, compiled the plugin, and got an .so file, that Claws was able to see?

Yes, that's fairly accurate. You need a few other dependencies, such as compiling claws-mail itself as well as an additional library or two. But once those steps are done I just copied the library over to the phone and loaded it with claws-mail's plugin dialog. This is obviously meatball-surgery, but I'm used to it. =)