@Kumasasa - yes the new launcher fixed the problem for me in single player mode. Is there a server version of minecraft-snapshot-13w19a? I play on a local LAN in my house.
I have this same problem - like Michael Barrett it was intermittent in previous versions but now in 1.5.2 MC is completely unplayable. I walk 5 blocks and completely lose keyboard.
Hardware:
iMac 27-inch, Late 2009
Processor 2.8 GHz Intel Core i7
Memory 16 GB 1067 MHz DDR3
Graphics ATI Radeon HD 4850 512 MB
Software OS X 10.8.3 (12D78)
@Kumasasa yup that seems to have fixed the issue for now.