I have yet to see any A700 user tests of newer 600x cards (hence the "suspect" part of my post). ;-)
Rob does have a few tests of camera using newer 600x cards in this article:
But, it doesn't look he's tested them in any Sony dSLR models yet. FWIW, it does look like the Sony A700 has faster write speeds to media using a 300x card, compared to the new Nikon D300s using 600x cards. The fastest results he's posted so far for write speeds to newer UDMA 6 capable cards came from a Canon 5D Mark II (clocking at 45.8MB/Second to a Photofast 533x 16GB card) . But, I suspect some of the other cameras around may be able to outperform it for write speeds to media. We'll have to wait for test results using these new cards with more cameras to find out.