In reply to stp:
> I don't think trad climbing will be any different to any other kind of climbing.
I'm not sure about this, I think different ability at types of climbing relates pretty closely to what you train.
Firstly, a lot of trad climbing is about not getting pumped. It's not strength, or power-endurance, it's the ability to hang around in semi-strenuous positions placing gear and working out moves, for ages. Pembroke climbing, for example, often requires very few if any hard moves - it's all about not getting pumped. I've trained stamina (4x4 routes) all winter with just a few bouldering sessions to maintain finger strength to great effect for increasing my grade on these routes. It doesn't matter how well you boulder - if you get pumped, you fall off! I don't believe a bit of traversing is going to do much for spending an hour or so on an overhanging wall of pretty big, possibly greasy holds, placing gear, and managing the pump. Onsight trad on steep 40m pitches requires a lot of stamina, and not a lot else.
On the other hand, for natural grit routes, doing 4x4 routes isn't going to be any help at all, and just bouldering is probably your best bet.
Back to the OP, I find it hard to train stamina in the bouldering wall, but that's just a boredom-threshold thing. Get on the circuit board in stay on it for ages (in a systematic way) if you're training stamina, and train strength by doing hard problems too.