I did some digging around and found the problem. Firstly, some weeks ago I tweaked the configuration of my webserver such that the WPAD URL - viz. http://wpad/wpad.dat - no longer worked. This is the URL Windows clients use when you tell them to automatically detect proxy settings, and by extension I tend to stuff it into anything that claims to support proxy autoconfiguration but doesn't actually support fully automatic detection. I didn't notice I'd broken it, as everything using it decided that a 404 on this URL simply meant to abandon any attempts to proxy. It appears that some combination of the 10.5.3 update and iTunes 7.6.2 now causes iTunes to fail at network access unless the proxy autoconfig URL resolves to something useful. Having fixed this, iTunes is once more happy. I'd update my iTunes bug report except that, er, iTunes' feedback system doesn't allow me to refer to my previous bug reports, apparently.