heh, found it myself later:

for others - use this macro:

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
With Target
If .Count 1 Then Exit Sub
If Not Intersect(Range("A2:A10"), .Cells) Is Nothing Then
Application.EnableEvents = False
If IsEmpty(.Value) Then
.Offset(0, 1).ClearContents
Else
With .Offset(0, 1)
.NumberFormat = "dd mmm yyyy hh:mm:ss"
.Value = Now
End With
End If
Application.EnableEvents = True
End If
End With
End Sub

Found on http://www.mcgimpsey.com/excel/timestamp.html.