2013 Feb, after discussion, mining.subscribe was extended to take 2 arguments, so now it's mining.subscribe(String useragent, sessionid) where sessionid is optional, but if provided must match a previous session's mining.notify subscription id. If the server allows resuming a session, it will give you back the same subscription id and extranonce information.

BFGMiner (at least) will also send mining.suggest_target("hex target") upon connection, if the user has a preferred target.