Yes the first models are being shipped the 14th of May. That's tomorrow
The advantages of the T42 in general, is that it comes with a Dothan cpu, instead of the Banias one in T41. The Dothan is 5% faster and uses less battery - you can find a review here on toms.
Most new T42s have also been upgraded with a Radeon 9000 instead of a Radeon 7500 - and some with a Radeon 9600 instead of Radeon 9000. Furthermore, some top-end models now come with an ultrabay slim DVD-RW drive.
edit: Try looking at forum.thinkpads.com - a lot of info on the T42s to be found there.