I'm using Mongoid with Sunspot Solr, and I frequently update the synonyms.txt file, but I'm not sure what method triggers the index to update using the new synonyms file.
Right now I do Sunspot.index(Model.all), but that doesn't seem to be working properly.
You might want to look at this: http://groups.google.com/group/ruby-sunspot/browse_thread/thread/5edee323a9833316
From the looks of it, the easy answer is that you can restart solr and that should do it.
Alternatively it's documented that solr can be configured to use multicores here: http://wiki.apache.org/solr/CoreAdmin and when using a multicore setup you can issue a RELOAD ( here: http://wiki.apache.org/solr/CoreAdmin#RELOAD ) which will allow you to update without taking solr down.
I'm not able to test these right now, but if this helps and/or you find a solid answer please post here.