Its an apache requirement that the distribution contain LICENSE.txt and NOTICE.txt in the root folder.
For a Solr source distribution, this includes the underlying lucene source too (it is essentially a snapshot of our merged checkout).
So, for 3.1 my plan is to combine these manually to create appropriate LICENSE.txt and NOTICE.txt's that have correct references to this combined source checkout such that we are legal... but this is obviously not so great as then the stuff is duplicated: e.g. we will have dev/trunk/LICENSE.txt, dev/trunk/lucene/LICENSE.txt, dev/trunk/solr/LICENSE.txt, ...
If anyone has any ideas on how we can do this better for 3.2, that would be great.