This works for me. Assumes your comboboxes' names are indeed ComboBox1 through ComboBox100.
Dim cb As ComboBox
Dim i As Long
For i = 1 To 100
Set cb = Sheet1.Shapes("ComboBox" & i).OLEFormat.Object.Object ' Ouch!
MsgBox cb.Text
Next i
The .Object.Object trick I got from here.