First problem: The boot.img kernel can't read the supp.img disk. I did not try passing the option 'floppy=thinkpad' at the time, but I suspect that would fail because there are persistent problems reading the floppy drive under linux. I worked around the first problem by doing a CDROM based installation instead of a network installation.
second problem: The installation went smoothly until it came time to reboot. Then, the system froze when init (the first process) started up. Going on advice from someone concerning thinkpad 560's, instead passing 'mem=TotalM' I passed 'mem=(Total-4)M' to kernel. This worked. It's possible that '4' can be reduced but I haven't tried yet.
third problem: Thinkpad 600's use the neomagic chipset, so getting an X server was nontrivial. Once networking was working, I downloaded the XBF neomagic X server from redhat. I had difficulty finding a XF86Config file that supported 16 bpp @ 1024 x 768. Eventually, I cobbled together my own modeline and have it working succesfully.
fourth problem: Suspend mode. Whenever the laptop is closed it goes into 'suspend mode' which is some sort of low power consumption mode. Unfortunately, the BIOS does this in a manner which corrupts the kernel. The solution to this problem is turning off the suspend mode using the configuratin utilities that come with the laptop. http://www.levien.com/tp600.html has a solution.
fifth problem: The floppy drive. In order to make it work, you must pass 'floppy=nodma' to the kernel.
sixth problem: With a redhat-6.0 install, I had to add: 'exclude port 0x2f8-0x2ff' to /etc/pcmcia/config.opts in order to avoid a system hang when a card was inserted.
The Thinkpad 600 is a very nice machines, but it's a bit tricky to configure.