Unit 307 finally sold in August for 635,000, $35,000 more than the seller bought it for. I think this answers your question that yes people still flip properties in Back Bay. I think the real question is whether they flip properties profitably and that is not so clear. If you look specifically at Unit 307 you will see that the seller ended up with a net loss after commissions. I suspect the seller was not a developer looking for a "flip" but simply someone who had a life change, a new job, etc.. and was forced to sell the condo shortly after they bought it.
All that said their are successful flips that still occur. Buyers who are able to negotiate a below market deal thanks to a foreclosure or a seller in distress or developers who identify profitable ways to improve units like carving out a second bedroom or developing existing roof rights can flip at a profit.
Unit 309 was pulled off the market in October. Expect that it was either rented or will show up for sale again just in time for the spring market next year.
There is one bank owned listing in Back Bay right now (see link below). Worth a look if you are thinking about a flip.