Suffering from RSI, I have tried a few systems.
I don't know of any sufficient FOSS option at present, but keep a close eye on simon listens, which promises to provide full speech recognition functionality in future with a great interface. Currently it can only control applications (such as firefox), and cannot handle dictation. See here for preset simon 'scenarios' for controlling different applications: http://kde-files.org/index.php?xcontentmode=692&PHPSESSID=c5b8ecb5c140ab.... Also bear in mind that you have to install the HTK back-end for simon to use separately thanks to our Great British university of Cambridge licensing it under uncooperative terms.
Personally I use Nuance Naturally speaking 10.1 preferred under wine. It works on my Kubuntu 9.10 installation, but not on my Ubuntu 10.04 install (to my intense annoyance). It works pretty well on my Kubuntu install - I can dictate text fine for the most part. It doesn't interact with any other apps however. You'll probably need to use Wine Tricks to install an extra DLL or two but this is a simple task.
If you find any other solutions, please let me know! Simon listens is the great hope, and they desperately need resources, so if you can spare time/money/promotion then please hand it to their team.
Sam.