Apple would have been working on Mac OS X 10.7 before Snow Leopard was released. In fact in some ways they were likely working on it before Snow Leopard started being worked on - 10.6 was a tidy-up of 10.5, so many of the actual new ideas would have been put on hold for 10.7.
Like any software company. They have future versions in varius stages of development. While they're actively working on 10.7 now they no doubt are dedicating resources on 10.8
My hoped and realistic time-frame for 10.7 is as follows:
Announcement at WWDC.
Release Early/Spring 2011.
I'm hoping for an early 2011 release because Snow Leopard was minor update (for End-users). I am one of the few who can significantly notice the speediness of Snow Leopard over Leopard but that isn't enough to make me not want a true upgrade.
I'm hoping they release a nice demo (almost feature full demo) at WWDC with plans go releasing Jan-March 2011. I miss how OS X's release cycles were 12-18 months as it always gave you the latest technology at reasonable time yet it didn't go into a Windows XP syndrome (where users don't want to upgrade because they've gotten so use to the OS).
"Mac OS Y" was a joke (although the smiley did get lost somewhere along the line).
The previous Apple operating system was "Mac OS 9" rather than "IX", so there's no precedent that the next one will be "Mac OS XI". My guess is that it won't be. It's very likely that the upcoming operating systems will simply stay as "Mac OS X" for a while yet. We'll probably get 10.7, 10.8, 10.9, 10.10, 10.11, etc. until at some stage the actual underpinnings change and a new name is needed ... "Mac OS 3D" perhaps. 😉