Cate,

Try Range("A1").Validation.Value. From the help file:

"Returns a Boolean value that indicates if all the validation criteria are met (that is, if the range contains valid data)."

Thus, you could use the above property to check whether your changes are valid. If you capture the formula/value in a given range prior to making changes, then you could revert back to this original formula/value in the event that your code generates invalid data.

Ben