Personally, I think an Asus Transformer Prime running on Windows 8 will make for an interesting competitor.
The latest version has a 1920x1200 resolution display, which is close enough to 2048x1536, especially on a 10 inch tablet. The brightness was awesome, and Anandtech really liked the panel in terms of color reproduction.
The form factor, weight and build quality are all pretty good on the Transformer Prime and I would expect Asus to keep it up on their future releases.
The cameras were good as well, much better than the iPad 2's, but we'll have to see the iPad 3's cameras to know how they compare.
The battery life was a bit shorter than the iPad 2's, but the keyboard dock extends it to 15 or 16 hours.
Tegra 3 might be slower than the A5X, but then again it might not. Either way it's still pretty fast and runs everything easily right now.
Software wise Windows 8 looks very interesting, and we'll have the consumer preview in a few days. I've been playing around with a HTC Radar and WP7 at least runs very very smoothly. So basically the biggest downside will be app selection, but with the Microsoft store and the ability to run x86 apps, I don't know how long it'll take for MS to obtain a good amount of great apps for their platform.
Biggest value add for me though, would be the keyboard dock. I just can't type fast or accurately enough on any tablet for them to be useful right now
