Minor Description
Here's a patch that implements simple support of Lucene's MoreLikeThis class.
The MoreLikeThisHelper code is heavily based on (hmm..."lifted from" might be more appropriate 
Erik Hatcher's example mentioned in http://www.mail-archive.com/solr-user@lucene.apache.org/msg00878.html
To use it, add at least the following parameters to a standard or dismax query:
mlt=true
mlt.fl=list,of,fields,which,define,similarity
See the MoreLikeThisHelper source code for more parameters.
Here are two URLs that work with the example config, after loading all documents found in exampledocs in the index (just to show that it seems to work - of course you need a larger corpus to make it interesting):
Results are added to the output like this:
<response>
...
<lst name="moreLikeThis">
<result name="UTF8TEST" numFound="1" start="0" maxScore="1.5293242">
<doc>
<float name="score">1.5293242</float>
<str name="id">SOLR1000</str>
</doc>
</result>
<result name="SOLR1000" numFound="1" start="0" maxScore="1.5293242">
<doc>
<float name="score">1.5293242</float>
<str name="id">UTF8TEST</str>
</doc>
</result>
</lst>
I haven't tested this extensively yet, will do in the next few days. But comments are welcome of course.