There was a problem with the 1.5.1 version of WordPress that was causing RSS feeds to not validate.
It’s discussed in full here
I was able to resolve the problem by uploading the new wp-blog-header file.
In addition, in order for Bloglet to recognize the feeds correctly (i.e., not returning a string/date error), all I had to do was change the XML link from
http://www.blogname.com/feed/rdf/
to
http://www.blogname.com/feed/rdf
(without the slash)
Very odd that that would be a fix, but hey, it worked for me.