Okay, now that struck me (immediately!) as odd, that you don't get the gray Reader box with Wikipedia. So, here's a picture of my iPad1 screen after I did a query on Wikipedia. Now, it took a bit, because the site has to fully load, but the gray box came up at the end of the loading session. See here:
View attachment 8541
If you type in the URL from the picture and still don't get this Reader box, are on iOS 5.0 or 5.0.1, then may I suggest you make a backup of your iPad (to save your contacts and email information) and do a restore of your iPad. This is because, IMHO, there is something wrong with your Safari version - but you can't delete and re-install a stock app (you have to do the restore).
If you do a restore, you'll end up with iOS 5.0.1 on your iPad (just so's you know). But, you might/should get a properly working Safari.
Or - you could take the iPad to the Genius Bar at an Apple Store. Maybe they can figure out why it's not working...?
Sorry to not be of better help - but you not seeing the Reader button (on web sites I CAN) has me stumped.
Marilyn