We have no immediate plans to upgrade, or rather none have yet been proposed. Because major version changes in vBulletin have a habit of completely rewriting the templates, redoing them all for each community is an extremely long and draining task. There's also the other stuff that goes with any forum upgrade, which becomes quite intense when you've got the amount of data to deal with that we do.
There is a chance that we'll upgrade to 3.6 once it's fully stable and established, but it'll depend on what vBulletin's own plans are. The last thing we want to do is end up upgrading to 3.0 like before and then have 3.5 come along a few months later with sweeping template changes that make it unfeasible to upgrade.
That said, myself and Matt who dealt with the majority of the upgrade to 3.0 learnt a lot about the ins and outs of handling the whole thing, so hopefully it'll be quicker and smoother when we next upgrade. Rest assured though, keeping up to date is always a concern of ours.