I just upgraded to Tiger (yay, yawn) and, of course, Safari RSS.
In my first few hours of browsing with the new system I’ve received probably 5 or 6 instances of this happy message: Safari can’t open the page.

In almost all cases, I was working on Google’s AdWords site, and if any sites in the world can be guaranteed to stay up, you would think they’d be Google’s. Plus, I’m on broadband, so the probably is unlikely to be mine.
Very odd – I think Safari might just need to be set to be a bit more patient. Basically, what’s I think is happening is that it’s not receiving a response as quick as it thinks it should … and therefore reports a bogus error.
Other than this issue, the new version of Safari is very spiff, and I must say I’m liking it a lot. The RSS functionality is nice, although I kind of like Firefox’s a bit better … I can simply see the new titles in a menu in Firefox, and in Safari I actually have to kind of “go to a page” to see the RSS feed.
[ update May 27 ]I am getting this fairly often today as well … a refresh makes it go away, but it’s very annoying.