do note that telephoto converter used on digital cameras (mounted in front of the lens) is different from SLR teleconverters which are mounted between the camera body and lens (which could be changed for SLRs with interchangeable lens. cameras like G1 has a fixed lens). so if you intend to get a DSLR in the future, you may not be using the telephoto converter that you get now for the G1.
anyway, if you want a telephoto converter now, consider the Olympus TCON17 1.7x teleconverter (~$140). Get a 58-55mm step down ring which cost less than $10 (if you have the original Canon lens adaptor. you need either this or a third party one to attach additional accessory lenses).
AVOID all 2x or 2.5x teleconverters because for the majority of them, the quality is lousy and they perform badly.