I had the same issue, with the current version (188.8.131.52). Same solution: don't use commas. Quotes did not tickle the bug for me.
However, I'm still having a problem with the shortcode -- I can't get before_date to work. I tried this:
[RSSImport display="5" feedurl="http://members.swcp.com/webdev/feed/rss2" date="1" before_date="<small>"]
On the page, it displays the text "<small>" instead of leaving the tag there to have the intended effect. (I.e. the HTML source contains the escaped tag with ampersands)
I didn't find any notes about needing to escape tags. I tried several things to do that, but failed. Any ideas?