I have a little app that downloads stock prices and was working perfectly (for years) until my recent upgrade to 10.5.7. After the upgrade, the program would crash on this
At The Omni Group, we are seeing this new crash in 10.5.7 in all of our NSURL-using applications too, in asynchronous use (so I think the synchronous thing is a red herring). We've also had it happen in TextMate (presumably in the software update).
I've logged Radar 6932684 on this issue, and if you are seeing it too, I strongly suggest you all report it, with whatever details you can gather.