Actually, iTunes itself will work with the PSP for some music files once they start selling the EMI artists without DRM in May. This is because the PSP plays AAC format files without any problem as long as they are without DRM. As far as I know, all online music stores use DRM for their videos, so they won't work, although I believe video podcasts on iTunes are DRM free.
You can also look for any online site or service that provides iPod-ready video for free, since they should also work on the PSP. But you're never going to get easy iPod-like syncing with the PSP, IMHO.