Is there a reason why you all used methods from the Microsoft.VisualBasic for the built-in functions such as InputBox and MsgBox, but you didn't do the same for the string functions such as Mid, Left, Right, etc.?
Also, do you plan on creating a sub class for the different function categories (i.e. String, Date, Financial, etc.) or are you going to put everything in BasicBuiltins?
I have added Literal Dates (e.x. #3/4/2010#) and all of the Date and Time functions, but unfortunately I haven't had time to test anything.