I just found something interesting. I took a couple hundred photos and put them in a collection. I then went into the file Folders section of LR and toggled Pick status on a bunch of them. I then went to the collection and filtered for picked status and the changes weren't reflected.
To test this out I created two collections, "Collection A" and "Collection B", then added the same image to both collections. Flag it as picked in A, rejected in B. The flag status is unique to the collection.
This confused the heck out of me and I ended up just removing everything from the collection and re-adding it, but something I'll have to watch out for. I use "pick" status to indicate that I sent it to my Flickr account. That is true regardless of what collection it is in... maybe I need to rethink that a bit and just create collections that mirror my Flickr Sets and then drop images in there. That may make a lot more sense now that I think of it... hmmm...