We do have lots of problems with our Android Apps when using SDK14/ADT14. Currently we can\'t build none of our complex apps with lots of libraries and dependant projects (R
You need to make sure that you've updated both the SDK AND the ADT plugin in Eclipse - I forgot to do the latter and had the problems you've described.
SDK14/ADT14 introduced a lot of changes - the old ADT won't work with the new SDK and the new ADT doesn't work with the old SDK.
If you've done both and it's still not working then moyshe's link takes you to the relevant to stuff to download (just uninstall what you have already and install the older versions)