This evening, I found a good resource for RegEx during my entry validation quest. I was specifically looking for Improved US Phone Number (by Jesse Sweetland).
^1?\s*-?\s*(\d{3}|\(\s*\d{3}\s*\))\s*-?\s*\d{3}\s*-?\s*\d{4}$ recognizes 1-123-456-7890, 1 (123) 456 7980, 1 123 456 7890, (123) 456-7890, 123-456-7890, and so on, and makes sure that if one paren is present both must be present.
Some other good links for Regular Expressions in .NET