I love Vista's new ALT+TAB behavior, which is very close to the excellent TaskSwitchXP utility I used under Windows XP.
However, the default size of the Vista ALT+TAB live thumbnails is far too small on larger monitors.
But it is possible to increase the size of the Vista ALT+TAB live thumbnails -- at least to the width of your primary monitor -- via a small registry edit.
The necessary changes are below. Save this to a text file, rename it with a ".reg" extension, then double-click it to add the changes to your registry. If you don't like it, delete the /AltTab registry key and you'll get the old ALT+TAB behavior back.
Windows Registry Editor Version 5.00
I've also tightened up the borders and margins a bit, to make room for larger thumbnails. However, before importing this into your registry you must edit the MaxThumbSizePx to a size that works on your monitor first! The current value, above, is DC hex or 220 decimal, which works on my 1600 x 1200 primary monitor. The maximum row size is 7, so each row is 7 x 220 = 1540, right under the 1600 width.
I would prefer fewer thumbnails per row, for even larger and easier to see screenshot thumbnails in my ALT+TAB, but I can't find any way to change that, unfortunately.