[bti]

The way I do it is: map caps lock to Ctrl in System Preferences, then download this tool[1]. The setting you are looking for is 'Control_L to Control_L'. When you hold Control it acts as a modifier but if you tap it, it acts as escape.

[1] http://pqrs.org/macosx/keyremap4macbook/

[0ren]

More details can be found in the ctrl/escape section in:

http://stevelosh.com/blog/2012/10/a-modern-space-cadet/