Thanks guys for the comments, appreciated.
Most people's experiences from what I've read seem to be a short-term lift and then long-term back to square one - but it was mostly the content they were moving to the subdomain that they were interested in. The subdomain content got smacked down again after a Panda cycle or two.
For me it's the content left on the main domain that's important, and I'll probably just noindex the subdomain content just so it's at least accessible somewhere.
The main domain content has pretty good engagement and usability.
Has anyone seen any examples or have any direct experience about this way round?