1.8.1 is based on Freebsd 8.4. 1.8.2 is based on Freebsd 10. There hasn't been a lot of changes to the 1.8.2 branch except for a few fixes:
http://svn.m0n0.ch/wall/branches/freebsd10/CHANGELOGSo far those entries in the change log are the only modifications, except for the transition from 8.4 to Freebsd 10. I feel at this point the only reason to use the 1.8.2 is for testing purposes, or if you need support for newer hardware which Freebsd 10 may support.
I doubt you will see an official 1.8.2 release anytime soon unless there is a major security flaw that is found in the 1.8.1 branch. Luckily m0n0wall isn't vulnerable to the heartbleed security flaw, otherwise there probably would be a new release.