At first I didn't realize you were talking about a "smart" USB cable
with the electronics built-in.

A moment ago I looked at several electronics vendors web sites and
found more of the same. FOr example, at Frys.com there was such a
cable from Parallax for their STAMP system; it apparently doesn't
work with anything other than Parallax products (from the description)

Same for Garmin; their USB to RS232 is only for their GPS products.

And everyplace else I looked, same story.

Getting a generic USB-to-RS232 convert box seems the best solution.

There are two other solutions:

1. if your system is a laptop, there are PCMCIA cards that provide a
true RS232 interface. THese seem a bit pricey, though.

2. if your system is a desktop and has a spare PCI slot, there are
many 1-/2-/more-port RS232 cards that seem reasonably priced.

I just took a look at the last 3 computers I bought and none of them
have RS232 ports, so I'm going to buy 1 or 2 PCI cards so I don't
lose the capability of working with RS232 devices. All my Dell laptops
have RS232 ports, and a new one I've been lusting after (a Latitude
D630) has one also but its price is beyond my present budget.

Hah! I just went to Dell's site and in the reviews for the D630 one
guy wrote "I did not even boot this laptop to the installed windows
operating system. The first thing I did was install ubuntu."