A List Apart actually has a great article: Inline Validation in Web Forms where the best performing inline validation method actually always displayed icons after validation; a green circle with a check for success, and a red circle with an exclamation point for an error, plus red text for errors.
This method allows the icon to give immediate feedback for success and failure, and just by skimming the column of icons you can see your errors (the only ones with text). The success icon gives some added confirmation that the field was filled in and checked out.
When compared to our control version, the inline validation form with the best performance showed compelling improvements across all the data we measured. Specifically, we saw:
a 22% increase in success rates,
a 22% decrease in errors made,
a 31% increase in satisfaction rating,
a 42% decrease in completion times, and
a 47% decrease in the number of eye fixations.
Here's a Video Demo of their inline form validation that performed best in tests.
Note for a form with a large number of optional (or only optional) inputs, showing successful validation may not be helpful.