No they haven't... My mate sam got a new 360 elite for xmas just past and it died on him last week... So i guess not all problems have been sorted!
As far as I can tell from the SA sample, the new units fail at approximately the same rate as the old ones. That is, nothing has been fixed yet and as far as anyone can tell, Microsoft hasn't done anything to address the 3RROD error and has quality issues on several other areas. For example, the DVD drives have a high failure rate and commonly scratch discs.
New "falcon" 360 units have their core CPU fabbed by IBM with a 65nm process. That draws less power and runs a little cooler. Also, MS has added a second heatsink connected to the primary GPU heatsink via a heatpipe. Neither of these changes seem to have a significant effect on the reliability of the 360.