If you’re using responsive techniques (such as Philben suggested) then Tablets will get the desktop layout for better or worse. I have to ask WHY you want to ignore Tablets, as it’s not that much more work to implement. Trying to ignore them would be a lot more work as you would be using adaptive techniques to design a mobile layout and a desktop layout and then try to sniff for only those devices.
Hmmm… after re-reading your post, are you trying to make a mobile only site? Either way what are you going to do with visitors that are not mobile or mobile/desktop? Or have you already got the “desktop” site done. In that case I would do as andy said above and use a framework like jquery-mobile or Sencha so that the mobile site is as cross-device compatible as possible.
I would argue that iPad falls in the “mobile” category as well.
A site can not behave differently based on the criterium “mobile” or “tablet”, since those are subjective terms.
A site CAN behave differently based on objective terms, such as screen width, device width, browser type/version, OS, etc.
It all starts with the question what defines mobile for you, and based on that you’ll have to create your rules that result in showing a mobile site, or “regular” site.