Currently is seems that for any code change in a sails.js app you have to manually stop the sails server and run sails lift again before you can see the changes
For anyone coming to this question now, it seems that this is no longer necessary - an application launched with sails lift will have a grunt watch task running, and code changes will be visible without a restart.
I didn't realise this was happening at first because there's nothing to indicate what's happening in the console, but it does seem to work without a restart (I'm using Sails 0.11)