The reason for the clarification of section 3 is that what the FSF did to Mepis was inexcusable. (Further discussed in this thread.)

A small Linux distributor named Mepis (more or less a guy in his garage) partnered with a big linux distributor called Ubuntu (multi-million dollar company with offices in more than one country). Mepis put out a press release quoting Ubuntu's founder about how cool the partnership was, and then Mepis pointed to Ubuntu's source repository for GPL packages it was using unmodified Ubuntu versions of. And the FSF went after them.

As far as we're concerned, Mepis didn't do anything wrong, and the FSF was a bully. The FSF was wrong when it tried to make an example out of a company that was acting in good faith.

To make sure the FSF doesn't pick on anyone else against our wishes, we're clarifying that if you didn't modify the source code, and the binaries you're distributing can be entirely regenerated from a public upstream source, pointing to that upstream source in good faith is good enough for us. As long as the upstream source doesn't object to the extra bandwidth, and the correct source code stays available at that location you specify for the duration of your responsiblity to redistribute source, life is good.

There are a few common sense caveats. This doesn't mean it's fair for a Fortune 500 company to point millions of people at somebody's home DSL line (certainly not without asking first). And if the source that's available there is not the complete corresponding source to the binaries you distributed, then obviously you haven't fulfilled your obligations by pointing to some _other_ source. (If you modified it, we want the patch, and claiming you didn't modify it when you actually did would be fraud.) And if the code stops being available at that location, you're not off the hook and have to find a new location or put up your own mirror.

So this is not a "get out of jail free" card: It's still your responsibility to make the complete corresponding source available. We're just saying you can reasonably delegate to something like Sourceforge or ibilbio, and as long as everyone who wants the source can get it, we're happy. If the site you point to objects or goes down, responsibility obviously reverts to you. But there are plenty of high-bandwidth places that mirror open source for free these days: sourceforge, OSL, ISC, ibiblio, archive.org, and so on.

Oh, one last note: if people come to you asking "where's the source" and your answer doesn't satisfy them, ask yourself "did I identify which specific version I used, and if I didn't modify it at all did I explicitly tell them this"? If you don't identify the source you used in enough detail for open source developers to reproduce what you did, you haven't complied with your license obligations yet. Identifying the specific source you used is a very important part of the "written offer" bit that often gets overlooked.