I have read many very complicated hints on how to enable passwordless SSH including this one here on macosxhints.
They are all just unnecessarily complex; here's the easy answer: Download the Gentoo Keychain and follow the instructions (very easy, you just add a line to your login script, .tcshrc or .bashrc etc.). From now on, you just have to enter your SSH key password when the first Terminal opens, and then never again until you restart the computer.
[robg adds: I haven't tested this one. Note that there are some other solutions suggested in the comments to the linked hint, which is actually the third of three in a series (1, 2). When I set my password-free SSH logins up, I used this older hint I wrote up, which is yet another way of doing this...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040428100632295