I'm guessing this is where Syd was heading, bit if it is a cavity block wall, (single skin wall with cavity blocks and not a double skin block wall with cavity) then they'll have to drill each and every chamber to ensure full fill. Not sure you'd ever see a return on this investment.
Leo
To ensure that each and every cavity block is filled would it not be necessary to drill each twice to fill each of the two cavities in a standard 18" block?* And if so would this not weaken the block and consequently the overall structure? This sort of work is very difficult to check to see that it's done properly and I can imagine cowboy operators doing little or nothing and then collecting the money since the home owner can't really do much to verify that things were done properly. Here is a recent thread which might be of interest. As mentioned above a search might yield more.
* Update: looks like I was on the same track as sydthebeat here!