Kolya sez, "BuyMate lets you SMS the ISBN of a book you're thinking of buying in Australia to 0427 767 763 and within around one minute will SMS the price details from four large Aussie book retailers and Amazon."
(Thanks, Kolya!)
Update: Tonx sez, "Google also does this. Simply SMS GOOGL with an ISBN number and it returns prices from Alibris, Powells, Amazon, etc."
Update 2: Nick sez, "we've been doing this on Txtbux.co.uk in the UK for nearly 3 years!"