How to change default ulimit values in Mac OS X 10.6?

launchctl has control over that now.

You can do a

launchctl limit maxfiles 2048 2048 

to set the soft and hard limits for maximum files open. For more information see the 'limit' section of:

http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/launchctl.1.html

There's an answer on StackOverflow for this one. Apparently ulimit does it now.