I don't think it is the PS Vita 2000 but a slimmer PS3 that might get announced. A Sony employee on the EU blog also mentioned no new software announcement at the event. Which makes me think it might not have anything to do with the PS Vita.
If they were able to get 8 PS3s onto a single motherboard for PS Now servers, it is possible Sony has significantly shrunk the size of 1 PS3. They also made adjustments to minimize latency and power consumption. It fits with the invitation hinting at the "slimmest" and not the "smallest" playstation product launch.
There's no way I can see this being another PS3 launch, they just launched the PS3 Superslim not long ago they aren't going to go and announce another PS3 again.
The PS Vita Slim makes the most sense, since for starters it already has the slim in it's name.
It's been a given it would be coming to the US/UK for some time now.
My only worry is we haven't seen any leaked store ad's talking about it which kind of seems odd.