Updating to a newer dev version of a module using Drush (ignoring point/recommended releases, without trashing version control)
I found your question just now because I had the exact same question. Didn't find an exact answer, but this worked for me: 

#svn rm &lt;module&gt;  
#svn commit -m"removing the module... whatever you want to write"  
#drush dl &lt;module&gt;-6.x-2.x-dev  
#svn add &lt;module&gt;  


For some reason, not doing the commit between the rm and the dl caused a problem with svn. I do not know enough about svn to know why, but would be grateful if someone could elaborate!