We've mentioned previous efforts to roll your own online iTunes backup using Jungle Disk and Amazon's S3, but frankly that seemed like a little bit of a hack, and potentially opened you up to some surprisingly large fees. Now Xackup is introducing an all-in-one iTunes online backup services called bandwagon, which uses S3 as well, but is mediated by Xackup, so the pricing is fixed at $69 year (introductory price) for unlimited storage. Xackup also includes a client, which sits in your menubar (right) and allows you to control the backup and restore process (e.g. selective backup/restore, search, etc.). Unfortunately, it appears that bandwagon is strictly a backup service and does not offer on-the-go access to your music (though I suppose in principle you could use it with your notebook to download particular tracks or albums you had previously uploaded from your desktop). Unfortunately, I can find no information on the bandwagon site concerning bandwidth limitations, if any.
As I noted before, bandwagon is presently available (until the end of February) for a special price of $69 for one year of unlimited storage, after which the price will jump to $99/year. If you want to try it first and see how well it works, they also have a 30-day demo of the service available for $1.
Thanks to those who sent this in!
[Update: the Xackup people have pulled the service because "the response has been overwhelming... [and they] underestimated the demand." They say their long-term pricing plan is "as yet undetermined."]