there was discussion here a year or so ago about postfix's recipient_delimiter. it wasn't possible under postfix 1.0 and waiting on upgrades to 2.0.
it (using "+" or "-") doesn't seem to be on yet. is it still the plan to enable these? that's the major hangup stopping me from migrating a bunch of domains over to dreamhost.
also, it'd be great if both + and - were accepted. on my current host, i do this with recipient_delimiter set to "-" and a rewrite rule for "+".