Oh what an excellent question! I was thinking the same thing!
I'm booked into a Birthing Centre and am hoping to use a combination of Calmbirth techniques and the water bath to help me thru. But I was speaking to a lady who recently had her baby at the same centre but couldn't use the bath cos her waters had broken so she used a TENS and said it was great (she wasn't using Calmbirth tho).
This got me thinking - what will I do if I can't use the bath? Has anyone else used TENS and found it useful?