Can I get a single feed combining all of my subscriptions out of Google Reader?
I mean I'm subscribed to 80 something feeds in Reader, and I want to use that as a single feed in another app.
Is that possible? If so how?
Create a bundle.
Within Google Reader go to http://www.google.com/reader/view/#bundle-creator-page ("Browse for stuff" --> "Create a bundle").
Then drag all your feeds into the bundle and untick the checkbox "Add to my shared items". Now when you save the bundle you will see it under "Your bundles" with 4 links next to it. One of those links is called "Add a link to your website or blog". When you click at that link you can see (and subscribe to) the atom feed of your newly created bundle.
Don't forget that at this point everyone in the world might be able to find out about all stuff you read.
I don't have enough points on this site yet to comment, but I was also going to suggest the same thing that Grant did.
It depends on exactly what you're trying to do, but using the OPML Aggregator Pipe you can just point it at your Google Reader OPML.
If you are going to use apps like FeedDemon, you don't need to combine all the feeds. It will automatically import all your subscribed feeds from Google Reader.