MedPDA.net used the newly launched PubMed feeds to get RSS for any journal. He also explains how to set up the feeds yourself in the post How-to: RSS feeds for medical journals.
NEJM and BMJ offer their own RSS feed but others, at least as far as I know, do not.
JAMA and Archives Have RSS Feeds Now
PubMed RSS feeds are now live
Getting RSS feeds for most medical journals - MedPDA.net
How-to: RSS feeds for medical journals - MedPDA.net
Use Plucker to get Journal RSS feeds in your PDA - Palmdoc.blogspot
RSS tutorial for STFM Board. Family Medicine Notes
A really simple guide to a powerful tool: RSS. BMJ Career Focus 2006;332:244.
How To: Create an RSS Feed for a Feedless Journal with PubMed. DavidRothman.net.