John Samson ansered "In short, yes." but the answer is "In reality, no."
I realize this is old, but I ran into exactly the same situation as the original poster and found this posting and used the short answer above to start working on it. I got an error that explictly said I needed SQL 2008 or later as the publisher to use SQL 2008 Express as the subscriber. This MS page:
http://msdn.microsoft.com/en-us/library/ms143241.aspx
clearly states it in the matrix of publishers and subscribers:
" This configuration is not supported because the Publisher version must be equal to or greater than the Subscriber version."
Anyway, I wanted to post the clarification in hopes that it will save the next person the time and frustration it cost me.