So I just ordered a Withings wifi scale, in the knowledge that I wouldn't be able to get the data into Garmin Connect. So I went ahead and created a very simple crude tool to import the data.
Its a command line tool currently, for Mac OSX. It is very crude, but I put it together this afternoon. I plan to add a front end to it, then eventually fully automate the process, but in the meantime, if it helps someone, here is a link to the tool (Intel only). I have only tested it on OSX Lion...
http://bit.ly/withingsconnectosx
Usage from the terminal command line:
withingsconnect <output fit filename> <weight (kgs)> <body fat (%)> <muscle mass (kgs)>
i.e.
withingsconnect weight.fit 90 25 50
Once the fit file has been created, you can manually upload it to the garmin connect site.
Cheers
Jonathan