Is it legal (either in the general sense or within the App Store) to use publically available XML feeds w/o contacting the original author?
For example something like http://earthquake.usgs.gov/eqcenter/catalogs/7day-M2.5.xml
(Not really what I'm using for my app but same type thing)
IANAL, but I would say yes. The data is already out in the open, in a machine readable form (i.e XML) so I would say go for it. There are a lot of apps in the App Store that are essentially just a wrapper around public XML/RSS feeds and web services.
I would suggest that you definitely contact the original publisher of the data. I believe there was just a big snafu of ex**actly** this situation. Someone used free data published by their city about parking spots or something like that.
Long story short, the city sued them and I believe they had to stop using their data feed.
It all depends on the license attached to the content, and you probably can't find it from an XML feed without contacting the original author.
IANAL disclaimers apply.
Edit: I can NOT find the article I'm referring to after a few searches. Doesn't mean it's not there, but you may want to just disregard this info unless someone else knows of the story I'm referring to and where to find it on the web.
Legal, probably -- as nstehr said, there are plenty of existing usages that back that up. (And that feed you point to is a US Government work product, so by definition it's perfectly valid to use.) Of course, the wrinkle of the App Store is that if Apple decides your particular usage is verboten or otherwise unkosher, you've really got no recourse... so you have to consider that.
Whether it is legal is dependent on whether the data is protected under any form of licensing. If your app makes it big and uses a lot of their bandwidth, they could potentially sue you for using there resources.
The best thing to do is contact them and ask them if you can use it. Perhaps try to arrange some sort of deal with them.
PS. IANAL
Good responses so far, but one thing not yet mentioned is that your XML feed can also disappear in a moment's notice since you don't control it. Determine what you think the risk of that is, and if your app would stop functioning and/or malfunction without it, consider creating your own feed instead, anyway.