If you want to get it running quickly I strongly recommend Debian (if you have the luxury of choosing at this point). A large percentage of the developers use Debian and the Elane project seems to be using it for .LRN distribution as well.
Even if you do not use it for .LRN itself it makes installing all the supporting software a breeze.
Example:
apt-get install imagemagick (the BEST unix sysadmin I know once struggled for 3 days to get Imagemagick installed on a crusty Solaris box)
There are packages for everything you need now (e.g. Postgres, AOLServer, etc.)
If you decide to go the Debian route please help yourself and others by confirming and updating these instructions (post if you have any questions or problems):
https://openacs.org/wiki/Installing%20OpenACS%20on%20Debian%20Linux
