Up 66% percent, roughly the same rate that the entire market is growing. It's almost uncanny how the iPhone manages to maintain its near-constant share of this rapidly growing market.
I'm not sure if this is pedantic in context but actually Apple have been selling 3 or 4 phones for the last year. (The 3G was still available in some markets after the 4 was introduced just as the 3GS remains now in the US. Whether you count the Verizon iPhone seperately is debatable, but Samsung's American network exclusives seem to be counted that way, even if they share 99% of their make-up, though Apple has unified those now to some degree).
I'm thinking about picking up the 3GS here in Switzerland with a pretty cheap subscription plan, which is essentially a subsidized pay-as-you-go plan. (with a $20 subsidy over 2 years). So there definitely is a market for the older 3GS.
That's a different source though, and figures for the world not the US, so not directly comparable.
Neilsen do update the figures every so often in items on their blog, but that was the latest historical graph.
This entry from last month has the figures for June (so six months since the last figures shown on the graph) Android up 14 points to 40%, RIM down 8 points to 19% and Apple still steady at 28%:
I believe someone on hacker news was copying these monthly updates into a spreadsheet so they could make their own graphs without having to buy the full report and sharing the results, you might be able to find a link if you search.
A visual chart of this phenomenon:
http://blog.nielsen.com/nielsenwire/wp-content/uploads/2011/...