I have just got a replacement PS3 (slim) it's the 3003B model manufactured in July this year.
I noticed a few minor but notable changes; for instance the XMB runs smoother, now I don't know if this is because I may have had a faulty HDD (certainly there was corrupted data) but it is definitely seems quicker and more responsive for me.
Also it no longer emits that hideous cracking/creaking noise that the previous model was making as it warms up/cools down.
Finally and a little bit of a disappointment; it no long has back lit power & eject buttons. There is no blue light on the eject button so the only way to know if there is a game in the console is from checking the XMB (or hearing it running obviously). I kind of miss the white lit symbols on the buttons, they looked cool but no big deal !
The cracking noise is just random with some consoles, undoing the screws slightly stopped it. Never once heard mine do it, well not to the point that worries some people on here.