Wow forgot about this thread completely!
I guess I could give my 2 cents, for what it's worth. Reading - scirra.com/tutorials/73/supporting-multiple-screen-sizes over a few times was definitely helpful :).
Building your games in a bigger aspect ratio like 16:9 seems to be the way to go.
It helps to stretch your background past the layout size in-case a phone does have a different aspect ratio from 16:9.
If I want a sprite with a size of 100x100 I usually make draw it 200x200 and shrink it in construct, this allows it to be up-scaled if necessary.
It's difficult to do a once size fits all, especially for tablets with a HD display. However, when publishing to Google Play I believe they ask if your app is suitable for tablets.
Not a complete list of solutions by any stretch and supporting the vast range of screen sizes is a difficult task. I guess you could draw a non-HD version of the game and a HD version and maybe have construct check the resolution of the device and use the correct sprites accordingly.