Here's another answer. It works fast, reliably (see atomicules' answer) and has compact calling code:
' Returns true if item is in the array; false otherwise.
Function IsInArray(ar, item$) As Boolean
Dim delimiter$, list$
' Chr(7) is the ASCII 'Bell' Character.
' It was chosen for being unlikely to be found in a normal array.
delimiter = Chr(7)
' Create a list string containing all the items in the array separated by the delimiter.
list = delimiter & Join(ar, delimiter) & delimiter
IsInArray = InStr(list, delimiter & item & delimiter) > 0
End Function
Sample usage:
Sub test()
Debug.Print "Is 'A' in the list?", IsInArray(Split("A,B", ","), "A")
End Sub