Here is my "expert"

opinion: don't get the D600.
Here is my story: I had a D600 before this T41, and it felt very cheap. Ok, it's cheap, but it works, right? NO. Shortly after I sold it, the USB port stopped working on there and Dell took forever to figure out the transfer of ownership (and there's even an online form for that!). So then they replaced the motherboard and that was ok. Now, just after that, my other friend who had an identical D600 (he bought it after seeing mine - I converted him from a PowerBook, hehe) had his USB ports die too. So Dell comes and replaces the motherboard for his D600. Within a day, the replacement motherboard started letting off a smell of "burnt electronics" and died. When he called Dell up, they said to send it in (even though he has the on-site service). He sent it in, and
2 weeks later he got it back. We thought it was fixed and all, but the fun had really just begun.
He left it with me when he went to class, and using my l33t observation skillz, I noticed that even at 100% CPU load, it would only get up to 120 F, and I clearly remembered that mine used to hit 140-145 F. So I check the CPU speed, and guess what? They sent him a 1.3 GHz, but he had a 1.6 originally. He then got a T41 identical to mine (2379DJU) and called Dell and tried to get a full refund, since they had messed up so many times, but they refused to give it to him. Then within 2 days, he locked himself out of the ThinkPad and had to send it back using the 30 day no questions asked thing. Dell then sent him a brand new D600 to replace the old one.
So then, he used this D600 for a while without really any problems (except the build quality, of course). Then one day, it fried and wouldn't turn on. He had been lazy and hadn't sent back the old (faulty 1.3 GHz one) D600 so he got that out. THEN, believe it or not, the NEXT DAY THAT ONE DIED TOO!!!11 He was then left with 2 D600s that wouldn't turn on, and Dell support couldn't believe it, hahaha. He now has a T41p and has no problems with it (not exactly sure what happened with the Dells...I think his brother got it or something, heh).
Oh yes, and the friend I sold my D600 to still has it, but has had so many parts on it replaced the only thing that is original on there is the bottom of the chassis. The motherboard, LCD (and LCD backing or whatever), keyboard, palm rest, and hard drive have ALL been replaced (some multiple times). And the wrist rest still gets buring hot (that was the reason they replaced the HD, after he convinced them that an HD idling at 49 C is not normal). The keyboard is also very springy, as Nabeel said. The back of the LCD is also hollow aluminum so it flexes really easily and doesn't seem to protect the LCD very well at all.
EDIT: btw, I'm writing this at nearly 1am (ok, not that late but whatever) so there may be typos, and also, I am extremely satisfied with my T41. I have had no problems with it at all, and have only had to call IBM up once to get recovery CDs. As a current/former T41 and D600 owner, I would
strongly advise you not to get a D600 and to get a T4x. It's definitely worth the extra money.