In your excellent plugin that I have used for over a month now with no problems, I have a change request that might help SEO and getting your site re-indexed more often.
Why dont you have an option to not set the last date modified attribute when you create the google sitemap.
My blog has the witty text plugin that puts in new content each time the pages is refreshed. Why? My thoughts were that the search engines should see that my pages content is changed daily and therefore will need to come back and re-index more often. However, it you have the date of the posts put into the sitemap by default, and you have set the refresh time to be daily, then google will think lair liar as the last modified date of any post will not change if you have not updated that post. The other option could be to set all posts/pages timestamp to the current date/time. Im off to hack the code to do this at the moment.