I had this problem since the 2nd day I've owned it. I finally figured out the solution (I think, at least for me).
I have gfxcardstatus installed. I noticed that if I leave it on force integrated or dedicated and do a shutdown / reboot, I will have the gfx stuck at integrated the next time it powers on. And to resolve this I can only do a reboot once more for it to function normally and be able to switch between the 2 cards freely.
So the "solution" that I finally figured out after 2 whole months of playing around with it was that to set it to dynamic mode if you are gonna shutdown or do a reboot, otherwise u can set gfxcardstatus to force either cards and use it as per normal. Just remember to set gfxcardstatus back to dynamic mode before reboot / shutdown, and the next time u power on your rmbp you should not have this issue whereby it gets stuck on integrated card anymore.
Hope this helps. Cheers. =)