I have seen lowcost telephoto add-ons (under $100) on eBay that would double the Canon S1iS's telephoto out to 20x. On the CanonS1IS@yahoogroups.com mailing list, gerald48 reports his experience:
I found a 2x lens on ebay that sounded like a bargain, but like most things that sound like a bargain you get what you pay for. I purchased it, made by Merkury Optics. Guess I should have tried to check it out more thoroughly.
It is a terrible lens. At full telephoto, just the center of the subject is in focus, the rest is completely out of focus. I even set it up on a tripod, ( I turned the stabilizer off) and it was no better. Just thought I would let you all know before you think of buying one.
I once bought a 2x telephoto add-on lens for my now-defunct G1. Its flaw was that I could only use it at full zoom -- so every photo had to be a 6x photo. Any less, and vigenetting occurred (see the round outline of the add-on lens).