in order to do this, you'll need an event macro. See David McRitchie's
"Getting Started with Macros" if you need help:

   http://www.mvps.org/dmcritchie/excel/getstarted.htm

Here's one way: Put this macro in your worksheet code module
(right-click the worksheet tab and choose "View Code".

   Private Sub Worksheet_Change(ByVal Target As Excel.Range)
       With Target
           If .Count > 1 Then Exit Sub
           If .Address(False, False) = "A1" Then
               If InStr(.NumberFormat, ":") Then
                   Application.EnableEvents = False
                   .Value = .Value * 24
                   .NumberFormat = "General"
                   Application.EnableEvents = True
               End If
           End If
       End With
   End Sub

This will only convert the number if you enter it as a time. So if you
enter 8.5, the value will be left alone, but if you enter 8:30, the
value will be converted to 8.5.