I disagree that drop down menus provide accesibility problems, otherwise we wouldn't have them on Facebook, in every application Google has and in Yahoo as well and well honestly the list goes one. The reasons behind the use of drop down menus has been covered previously and extensively.
As for the tables, the problem was the images within them that was causing the problem. This has been sorted out now in the latest SVN. These are now text links for now as there are several areas that we need to consider before any else.
Not sure what you're reffering to with regards to centered panels? Are you talking about the entire admin? My original thoughts were different about this, however a fluid design looks completely wack on larger screens which more and more people are buying. Yes we shouldn't be driving away people with smaller screens (especially mobile screens imo) but the majority of users (and there is loads of evidence online to support this) will be at least 1024 and above. This does of course open the debate to whether or not we go 800 wide and cater to everyone but we're not really pushing the boundaries of using the available space? Do we go for a standard 1024 and then we don't have the native support for 800 wide? Do we go for a fluid design and make anyone with a large screen feel that everything is over stretched?
My personnal perference and it's completely up for discussion nothing is ever set in stone is 1024 as a minium. I originally wanted to use the max-width attribute but alas IE doesn't support it.