For a couple of years (!) now I've been looking for a replacement that would meet my needs. The trickiest requirement is an IRDA port; Sony stopped including them after that model, and even larger laptops often don't have them. I can get another 505TX on Ebay for $400 in good condition, or much less if I get a parts machine (I'm more than comfortable transplanting components). It'd be a good opportunity to put a new disk in the thing, too. But I'd like to upgrade the OS at the same time as the disk, and that will require more main memory than the 128M at which the 505TX tops out.
Since I can't get IRDA and 256M in the same package from Sony, I've been looking elsewhere, with very little success. I've found it in some plastic laptops, but the last plastic laptop I had didn't even last a year, and I don't want to waste either the money or the setup effort. There are some nice machines out there, and I was close to giving up on meeting all of my requirements (which also include at least 1024x768 resolution, travel weight under 4 pounds including 802.11 card if that's not build in, and a not-too-tiny key pitch) so long as I could get something that would be durable.
So anyway, I was just now poking around review sites looking for models with metal cases and came across the Toshiba 2000/2010. Discontinued, but only recently. Magnesium case. Came with one of two hard drives, one of which had problems, but the 30M one seems to be OK. Nice screen, full size keyboard, commodity memory, good array of ports including IRDA. Built-in 802.11. Eighteen millimeters thick -- that's better even than the 505TX at 22mm. Now I just have to find one.