Is Snow Leopard now pre-installed onto MacBook Pros?

It will only have Snow Leopard installed. But you will be able to get Lion for free when it is released. See:

The Mac OS X Lion Up-To-Date upgrade is available at no additional charge via the Mac App Store to all customers who purchased a qualifying new Mac system from Apple or an Apple Authorized Reseller on or after June 6, 2011. Users must request their Up-To-Date upgrade within 30 days of purchase of their Mac computer. Customers who purchase a qualifying Mac between June 6, 2011 and the date when Lion is available in the Mac App Store will have 30 days from Lion’s official release date to make a request.

All Macs today come with Snow Leopard. The version loaded may or may not be 10.6.8 so you might have to do a free update to 10.6.8 to ready for Lion.

As soon as Lion is released you can download it for free since you purchase will be after June 6.

Allan

If the MBP comes with 10.6.7 (Snow Leopard), you'll need to download 10.6.8 (Snow Leopard) then 10.7 (Lion). If it comes with 10.6.8, you'll use your redeem code as mentioned and then just download 10.7.

It will have Snow Leopard installed. You can get Lion for free IF you want it.