Interesting... i hadn't heard about google's accelerator but if its as useful as their other software (in the later stages) i'll definately be trying it out when its done.
As for download accelerators, most just find extra links to dl files from on other sites so they are just using little tricks to speed it all up. Other than that they can open multiple downloads on the one server for one file, just simple things like that to make it all go a bit faster.

