FWIW, I just found an FTP to SFTP bridge called Tunnelier:
http://www.bitvise.com/tunnelier.html
That is allowing me to use FlashFXP, so at least I don't have to put up with an inferior SFTP client, but I'm still going to have to spend $40 on Tunnelier unless you guys add SFTP natively in the next 30 days.
I'd suggest you put some kind of note about this option on the web site - I actually only found it by accident, so a lot of people are not likely to ever even know about it.