Media Center should/will read in the new Folder.jpg covers... but you must clear out all the caches show above.
Even then you may have to go in and out of the the media center music library and scroll through your collection to give media center a nudge to starting reading the new artwork.
Also check all of your music folders for hidden system JPGs Folder, small album art, large album art filenames... if you find them delete them. In Explorer folder options select show hidden files and un-check hide protected operating systems files.
You do need to purge your collection of all these caches and hidden artwork files... so that all that is left is your lovely Folder.jpg files.
Step 3 is needed only for Vista Media Center not WMP11.
However, if you ripped all your music to WMV or MP3 using WMP11 then you won't need to do step 3 but it is unlikely that all your rips are as recent as Vista WMP11.
If you have MP3s ripped using WMP10 or a 3rd party tool then yes step 3 is necessary. Normally the artwork only gets shown for the first track... but sometimes it may not even get shown for track one... but another track instead... but typically it will only show for one track on an album.
The step3 fix cures this know bug by reading through your library and recreating the UrlToCoverArt.dat file used by Vista MC... let it open the existing file read through your library and then save the results over the top of the existing UrlToCoverArt.dat file.
It won't break anything ;)