The USB-rs232 converter option can work, but to be successful there are a few things to consider: first, you need to make sure that whatever chip you're using (FTDI, PL2303, etc) supports all the extra signals that you might need for your hardware. For example the simple IR blaster I use on my mythbox at the house is just an IR-LED connected to the DTR line of a serial port (with a diode and maybe a resistor, I'd have to double check now I built it a while ago), and the LIRC serial module just turns that DTR line on and off at the appropriate rate, so for the tablet using a similar approach would require a chipset that will give you proper DTR control. I believe the FTDI and PL2303 will, but never really tried so you'd have to double check.
The second problem for the direct serial version like I just mentioned is that the range is very small; on a regular PC you'll get maybe a foot or two tops, and that's relying on the 12V signal line to drive the LED; on the USB converter the signal lines are usually only 5-6V (depending on the converter, some actually do put out 12V but most don't) which gives even lower performance. Now, there are better circuits out there, but I haven't tried 'em... the biggest issue though is that in order to attach the USB/serial converter you've got to have the kickstand down so you can get to the USB port, which is not always very convenient.
If I get some time I'll try taking a crack at building the IR blaster for the headphone jack and looking into that again too...

I managed to find a reference to the old Griffin Technology product (they discontinued it about 2-3 years ago now, but the patent does a decent job of describing how the thing worked):
http://www.freepatentsonline.com/6931231.html
As long as they don't decide to try and sue us or anything ;-) I know there's a better document around describing how to actually generate the waveforms and things that I'll post up a bit later (I've got it bookmarked on my PC at home but don't have access to it right now).