I got all my data back last night by simply adding a new item to my Health dashboard (Health Data > Show on Dashboard) and looking at the settings for all the items I'd already got showing on my dashboard (not changing any of them), after that I noticed all the other missing data was showing on the dashboard again. I'm not sure which step made them reappear and it may have been coincidence of course, but that's all I did - no resetting anything, no swapping sources in Health.
As an aside, I know it's only a couple of days since iOS8.0.2, but where are all the app updates? I had assumed that the likes of Nike, Jawbone, Withings, Fitbit etc would have had their new apps ready weeks ago and that Apple would have rushed them through approval to showcase HealthKit, presumably ready for iOS8.0.0. So 10+ days down the line where are they?