Well I bit the bullet and just ordered one from RC Champ. I couldn't resist at their price - it worked out about AU$50 cheaper than what I could get a B4 for overseas and about AU$200 cheaper than B4 at local prices
With regards to what wheels will fit - I heard that there will be an optional axle to run B4/Losi style rims on the back.
Not sure about the front though - even if you could put a Losi-sized hex on the front (the one on the front of the 501X would do) you would still have to drill out the centre of the Losi XXX rims to fit over the 4mm axle.