If you mean "hover" as in the mouse is used to move the cursor to hover over the field, then this may not be compatible with touch-based (non-mouse) interfaces like an iPhone/iPad. This may affect PCs users with touch-sensitive screens (e.g. HP TouchSmart PCs) in that they are forced to use their mouse. This may be mitigated somewhat if the error message is still shown when the field has focus, not just when the mouse hovers over.
Interestingly, Steve Jobs mentioned something about "hovers" and "rollovers" recently in his rationale about why iPhones/iPads won't support Flash:
http://www.apple.com/hotnews/thoughts-on-flash/
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Fifth, there’s Touch.
Flash was designed for PCs using mice, not for touch screens using fingers. For example, many Flash websites rely on “rollovers”, which pop up menus or other elements when the mouse arrow hovers over a specific spot. Apple’s revolutionary multi-touch interface doesn’t use a mouse, and there is no concept of a rollover. Most Flash websites will need to be rewritten to support touch-based devices. If developers need to rewrite their Flash websites, why not use modern technologies like HTML5, CSS and JavaScript?
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