Sigma discontinued production of the old non IS bigmas a few months back and they quickly disappeared from store inventories. The new IS bigmas are heavier, much more expensive (~$1500 vs ~$1100 USD), and available on all mounts except 4/3. I'd imagine the folks at Sigma rightly concluded most 4/3 shooters weren't going to kick down the extra cash for redundant IS.
It is a real loss to 4/3 shooters. There's simply nothing else on the market to replace it at this time. The good news is that you can get a 50-200 SWD & EC-20 for around $1200-1300. That gives you a superb fast tele for sports, events, and even portraits + a decent 400mm @ f/7.1 for birding, surf, lunar, etc.
I do hope Oly will introduce a high grade long zoom to replace the Bigma. I'd love something like a 200-500+ f/4 for less than $2K. I'll keep my fingers crossed...
I second what everyone else has to say about the 620. It's a great body, and a deal for the price. I'll be using two to shoot a wedding in two weeks. The articulating screen is very useful, backlit keys are a nice feature, UI is getting better with every body.
Main downsides I found on the 620 vs my old 510: The sound of the IS on both my 620 bodies is louder and much cheaper sounding than the 510. Mirror slap also seems louder and more pronounced. Finally, even with the HLD-5 I find the body uncomfortably small, whereas the 510 with 3rd party grip was perfect for me. Speaking of the HLD-5, Oly gives you a dial, but left the EV comp button off the grip. Lame! You gotta step up to the E-30 for that feature I suppose...
I suspect we'll see the 4xx and 5xx lines die off replaced by the 6xx series, E-30 replacement, and E-3 replacement. Exciting times in 4/3 world. Oly's gotta deliver the goods and produce a pro camera that's not only competitive to the market now, but 18 months from now. I can't wait to see what happens...