We’re going to take this one from Macrumors and Applerejectedme.com at face value. Apple registered iSlate.com through their third party domain registration/hiding service Markmonitor.com in 2007 when it likely purchased the domain from a company called Eurobox. There were a few weeks in 2007 when Apple was the domain holder.
“I’m hoping we can get the newsroom more actively involved in the challenge of delivering our best journalism in the form of Times Reader, iPhone apps, WAP, or the impending Apple slate...”
Techcrunch does some more investigation on ‘iSlate’ domains and finds out that Apple likely registered these all over Europe in 2006. It would seem that Slate or iSlate would be the frontrunner in choices for the new Apple tablet at this point, though lots of things could have changed between now and 2007. Interestingly, this was about the time that Apple unveilled the iPhone.
Update: a commenter