The GeoTools project has a policy of tracking and taking advantage of the latest developments of the Java platform.
However, we recognise that companies, groups and universities take time to adopt and install new versions of Java, either because of the time involved to do so or because of reluctance to use untested technologies. For this reason, our policy states that we will not move up to a new version of Java until the first minor release comes out.
For example, when the next version of Java comes out (1.5 at the time of writing) we will not start using features from it until version 1.5.1 comes out. This usually takes 6 months and, by this point, most major bugs or quirks will have been resolved.
If you need to develop against older versions of Java, then you might want to take a look at an older version GeoTools - notably GeoTools-lite (lite.geotools.org), aka GT1, as this works with all versions of Java from 1.1 onwards.