The 360/PS3 have been out for years, but their NDAs are still in effect... I'm careful when discussing their hardware to only mention details that are already present on wikipedia.
But now they should be able to [break NDA] because the console is out now.
The WiiU fits in a similar performance bracket to the other consoles, except it can push more pixels and has more RAM, but might have to move as much CPU-work over to its GPU as possible in order to cope with its relatively slow CPU
CPU GPU RAM(size) RAM(speed) #1 PS3 WiiU WiiU 360/PS3* #2 360 360 360/PS3* PS3/360 #3 WiiU PS3 PS3/360 WiiU ... #4 Wii Wii Wii Wii <img src='http://public.gamedev.net//public/style_emoticons/default/sad.png' class='bbc_emoticon' alt=':(' />*hard to pick winners in 360/PS3's RAM. PS3 has the annoyance of being physically split, but in some situations this can actually be a good thing. Overall, they're close enough.
Expect the PS4/"Xbox720" to leave it in the dust, performance wise.