I found a cheap USB to RS232 cable on eBay. It definitely looks a bit cheap with translucent blue ends and silvery cable, but no problems with quality. Ideal if you have a project that uses serial TTL.
In my case I managed to get a display and GPS sharing the remaining serial TTL com port. The display needed TX only. The GPS needed RX only. I just had to ensure they expected the same baud rate and all was good.
The mini can be programmed and debugged over either COM port but that port is then unusable whilst debugging. Should be no problem using both in your application without debugging too.
Chris has also hinted there might be a version of the mini with 2x serial TTL at a later date. I'm sure Secret Labs have plenty of other things to keep them busy first though.
The issue is really that if you have to reset and flash the mini you can only succeed on the RS-232 port (com1).
I'm going to look for a "real" usb to RS-232 adapter. If I find it I'll post it for other people in my situation to look at.
On a happier note my new mini arrived and it works perfectly!