How do I change the default boot option on rEFIt?
You will need to edit the rEFIt configuration file. Since version 0.14, the configuration file has supported adding a default boot selection, as documented in the news section of the website.

However, there isn't very clear documentation on how exactly to do this on rEFIt's website. You could just open the conf file, or view the default on their SVN repo here. This blog does have an example and some documentation that I've used before in a similar situation. You'll want to use the digit option instead of the OS type option (1-N, vs L,W,M).