I did a clean install of Windows 7 and I have to say that it runs really well - far better than Windows XP ever did with a similar configuration. Superfetch makes a HUGE difference. I haven't tried to add ReadyBoost yet.
Although the system initially came up with the standard VGA display driver, the Radeon 9600 driver was automatically installed the first time I connected to Windows Update.
The Aero interface did not work at first. I'm not sure exactly what I did to get it working - maybe generating the Windows Experience Index under Control Panel / Performance Information and Tools. I also rebooted a few times. In any case, Aero is now working perfectly.
Here are the numbers reported by the Windows Experience Index:
Memory (RAM): 4.1
Gaming graphics: 3.6
Hard drive transfer: 2.0
As near as I can tell, all features of the laptop are running properly. There are no missing drivers in the device list. Speedstep works properly. The wireless card works (although I had to choose WPA instead of WPA2 to connect properly to my WRT54G router running DD-WRT.) Display brightness works, volume works, even the ThinkLight. The only thing I installed from Lenovo is the Active Protection for the hard drive.
All in all, I'm quite happy with the T42/p and with Windows 7 experience running on this system.
[Update 5/21/2009] I wiped the system and installed Windows 7 RC today. I confirmed that Aero kicks in AFTER you generate the Windows Experience Index and reboot.
Here are the numbers reported by the Windows Experience Index for the RC. Compared to the Beta, Processor and Hard drive transfer went up and Gaming graphics went down.
Memory (RAM): 4.1
Gaming graphics: 2.0
Hard drive transfer: 4.3
[Update 10/28/2009] Lenovo has published System Update version 4, which provides a few updates for Windows 7, including the HotKey manager for onscreen feedback of volume, brightness, etc. ActiveProtection is also supported. However, most of the other Thinkpad utilities are still not available for Windows 7.