looking through both the Lion and iOS 5 feature pages on Apple's website, i found nothing mentioning that the reading list could be viewed offline.
The difference between the reading list is you can save articles you wish to read, and it displays which ones you have read and which you haven't, it also displays the first few lines of the article so you can tell which is which.
other than that it's pretty pointless, I would of liked it to be separate from my bookmarks, as i have iCloud syncing of bookmarks turned off, because some bookmarks on my Mac, I have apps for on my iPad/iPhone and are therefore pointless wastes of space, but this also turns off the syncing of the Reading list, which if they were separate would make it a lot more useful, as it would mean yo could safe something to read later on another device without messing up your organised bookmarks.