The <mx:Form/> version presents a red tooltip error message to the right of the item line which overlays anything it may cover. The <s:Form/> seems to only want to present a text message to the right of the item line and pushes over anything that may be close to the item being validated. In general, this is ok if you have a vertical form but TERRIBLE if you have a horizontal form. It pushes all the items to the right of the one being validated over the length of the error message. It really scrambles the screen.
Is there anyway to still use the <s:Form/> and have the error messages appear in a tooltip, popup or something other than the current text block that pushes other components around on the screen?
What you would need to do is create a custom version of FormItemSkin that disables the errorTextDisplay or repositions it somewhere more appropriate.