Wow, I haven't heard mention of this for several years.
Back when this camera was current, plenty of people did 'the hack' and I don't recall any major problems.
For those that don't know, when Canon released the original Digital Rebel (300D) and the 10D...they basically had the same operating system...but rather than adding to it for the upscale 10D, they just disabled some things for the lower-scale Rebel. Some resourceful geeks found a way to re-enable the features that has been turned off, thus making the Rebel behave much more like the 10D. (obviously the main difference is still in the size & ergonomics of the body).
So while you have to assume all the risks, I'd feel pretty safe in recommending that you try the hack on your 300D. Although, if I was to make a better suggestion, I'd say to trade that thing in for a 20D or 350D etc. There was a pretty big step up after the 300D & 10D.