It would be really handy to have the option of a currency item, such as industrial credits as well as the virtual money. The trade signs are good but sometimes it makes more sense to use something like the trade-o-mat or, I believe one of the 1.5.2 FTB packs has a vending machine. I assume with the past vote for industrial credits problem that you used a bukkit plugin to remove the recipe, with limited success? IC crafting can be disabled in the industrialcraft config. Having the GregTech credits as well would be ideal because that gives a wider range of currency values.. There should also be infinite buy/sell signs for the credits so they can be easily traded for virtual currency.
The issue with the credits wasn't the crafting It was more they would get one and dupe the credit As for the vending machines once the update happens.. I believe that it's not installed from standard meaning every player to join our server would have to manually go and change it, which can be done but would be hard/annoying Although using items rather than money is something to be thought about for when it's update time
It was tried and failed because some F*** wits duplicated them and spent them to buy a quantum generator. So i don't think it is really feasible.
For clarity, this suggestion is aimed at the 1.5.2 version, not the current 1.4.7 version I can see the point about duping but I still think it's worth considering because, hopefully, the dupe bugs are fixed in the newer versions of offending mods and also duping is duping; there's not much difference between duping the coins versus duping items to sell. As for the vending machine, I didn't remember the manual install part but yes, custom client would be more hassle than it's worth. Even without vending machines there are still trade-o-mats.
Although the problems still remains, there will never be a sure fire fix to all dupes, glitches in the game, it is just too hard to do, that is why there are now electronic vote credits which can not be altered by anyone other than (im just speculating here) someone that is OP.