UPDATE: It appears this issue has been resolved on Apple's end. Several readers now indicate a properly functioning Software Update mechanism.
We have received numerous reports of Software Update having suddenly stopped working on Leopard, accompanied by a dialog box stating that "The file 'index-leopard.merged-1.sucatalog' couldn't be found on the server 'swscan.apple.com'." Many users are stating that they were able to do some updates (to iLife, for example) before encountering this roadblock. The dialog goes on to claim that this is a "network problem" and offers a Network Diagnostics button, but there is no doubt whatever among our informants that their network is fine, so don't be misled by that. Clearly the problem lies with Apple: either something is wrong on their server, or there is something wrong with Software Update such that it is looking for a file that doesn't exist (or both). It is to be hoped that this glitch is only temporary.