It's been explicitly stated that there is no quest or mechanic to increase your level cap to 60, it is simply increased automatically if you have the expansion registered to your account. The only mention of older content not giving XP was with regards to the host of level 50 dungeons that we have as they feared many players would simply farm them to get to 60 instead of doing new content. They will give no XP rewards after 3.0 launches but this will be restored at some undefined point in time later on.

The 2.x series MSQs will be giving new XP and loot rewards now though so it is expected that a new player will actually be around level 52 or so by the time they finish the 2.55 story (assuming they already had the expansion registered).