Wi-Fi and bluetooth wouldn't be that bad I guess. But the transfers of music wouldn't be half as fast as FireWire, 54MBPS is as slow as a guy in a mental hospital compared to 400MBPS that you get with FireWire. Bad move if it is in the next gen iPod. Plus it would have to be much bigger to accommodate BlueTooth/Wi-Fi, plus a bigger battery so that the battery doesn't lose any of its capacity.
And citystar2k, the fact that iPods have non-replaceable batteries is part of their charm. It's an all-in-one design, with very few visible screws or anything. It's why most people buy iPods. If iPods had an ugly side-panel where you could put in a new battery, it wouldn't be as popular because it would be ugly. Unless Apple engineers managed to put a side-panel somewhere else...