So far, people using copies of Windows XP that have been activated using one of the leaked keys have had no trouble getting patches and updates because Windows Update didn't check the validity of their licence. But as Microsoft is clearly stating that licence information is now being gathered by Windows Update, and that a "validly licensed copy of Windows ensures that you will receive on-going updates," then it seems pretty logical to presume that an invalidly licensed copy of Windows ensures that you won't.
Blocking the leaked keys however isn't likely to get Microsoft very far, because people determined not to use a legitimate key can simply use one of the KeyGen routines to produce a working key. This appears to be impossible to block right now, because the keys produced do seem to be indistinguishable from 'proper' keys, to the extent that they are, sort of, proper keys. In theory this could be stopped if Microsoft had a list of all of the keys it had actually issued, but it probably hasn't, and it'd be a tricky thing to keep up to date.
Source: The Register