This is the best I can do. It uses VB and is definately not ideal, but here you go:
Put this code into the code module for the sheet where you want to have flashing cells.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Value = 55 Then
For n = 1 To 10
Target.Interior.Color = vbRed
Delay (0.04)
Target.Interior.ColorIndex = xlNone
Delay (0.04)
Next n
End If
End Sub
Sub Delay(rTime As Single)
'delay rTime seconds (min=.01, max=300)
Dim oldTime As Variant
'safety net
If rTime < 0.01 Or rTime > 300 Then rTime = 1
oldTime = Timer
Do
DoEvents
Loop Until Timer - oldTime > rTime
End Sub
This will make a cell flash red if you type 55 in it. Obviously, change the 55 to whatever you want. You can also play about with the delay values to slow it down or speed it up. If you want it to flash a different colour, change the vbRed to whatever (there is a list in the VBA help somewhere for valid colours). If you want it to only happen for one cell, then add this to the start of the code:
if target.row=3 and target.column=4 then
(don't forget to close this with an endif at the bottom)
this works for cell D3. For a different cell, change the row and column numbers.
Note: this will only flash when the value in the cell changes to 55. To make it flash again, you must change it to something else and then back to 55. Hope this helps - like I said it isn't ideal, but its the only way I can think of.
Phil