The info I have:
1: If you just look at the jBPM core, yes, this would be feasable, most delay will be in DB access
2: 2 general sized servers (dual 3GHz, 8GB) would suffice is my guess. (I perosonally have no real figures, but I've done a small test on a lesser system with good results)
3: hmm..... That should come from JBoss guys or fellow forum users
The number of concurrent processes is no problem (just storage). The thing is how often they are acted upon. Tuning the DB (indexes) is also an important thing. The DDL scripts are generic.