9to5 Mac is reporting that the final touches are being put on a resigned MacBook Pro. So let's see if we can answer your questions with what's being said by "trusted" sources in Apple supply chain.

1. Will it be thinner ? - Yes, although it won't be tapered like the Macbook Airs, it will retain it's sturdier frame.
2. Will it have a Retina Display? - Yes and apparently the display will cost apple around an extra $100 per unit.
3. Will it keep the optical drive?  - No. Apple does't believe most people will need it and for those that do they provide a USB Superdrive for purchase.
4. Will it have USB 3? - Yes, Apple will finally include USB 3 as a compliment to Thunderbolt.
5. When can we expect it? Tim Cook has mentioned that Apple has lot to show us this year so all expectations are that Apple will unveil the MacBook Pros at the WWDC in June.

So is there anything else you should know ? Well, it looks like there won't be a dedicated Ethernet port, though you will be able to buy an adapter as you can for the MacBook Air. Also the sources are saying that the dedicated power button will move to the top of the keyboard just like the MacBook Air.