I dug up this text on their site: https://www.sparkfun.com/support#IntExportRestrictions, which says:
[...] These are products designed for civilian use, but that could be used for devious purposes in the wrong hands. [...]
So, I guess that tells me there are weapons or other "dangerous" machines that have UARTs. Who would have thought that, huh? :| Really dubious restriction, but I guess they don't have a choice.
On most orders for electronics parts, even if it's European distributors shipping to me, in Europe, I'll have to click some boilerplate where I assert to not make the chips available to terrorists on the Denied Persons List of the US department of commerce, which is conveniently available in ASCII...
That, and clauses about not using it to controller missiles. Which is admittedly more applicable than the weapons of mass destruction (exaggerated) clause in iTunes.
This SuperUser post talks about it: http://superuser.com/questions/365999/how-do-i-recover-data-...
http://howto.starahead.com/?p=61
http://www.overclock.net/t/457286/seagate-bricked-firmware-d...
https://sites.google.com/site/seagatefix/Home
I had backups, but still, it was nice to be able to do this.
There are modules that have variable I/O voltage, very handy when dealing with devices that have 1,8V, 2.5V, 3.3V or 5V logic: https://www.sparkfun.com/products/12731
Equipped with such a cable, you can also observe any errors that might occur on bootup, such as when you have made a mistake in configuring your config files on the filesystem.
For normal "Rasperry Pi"-Level work, the cheapest eBay-cable will probably be ok. If you have more exotic devices you'll appreciate UARTs that can produce almost arbitrary baudrates, or might support inverted Tx/Rx pins. I found some chinese knockoffs using a "CH310" UART/i2c/spi chip to be particularly broken.
(EDIT: Added last paragraph, fixed a few typos)