I've just been asked by a client about how their Facebook Fan page would look if they don't upload a cover by the 31st March.
They wanted to know if their thumbnails which currently sit along the top of their page will still show. But I don't think they will.
Instead, as pages without covers are still going to be converted to the new timeline layout, I think they'll just look like this:
Rather than like this with a cover:
So the page just generally looks more interesting if you do give it a cover, which should be around 850px wide by 315px tall.
The little row of thumbnails our client refered to is shown in the old screenshot below (the row of browser icons), but in the new timeline layout this row of your most recently uploaded photos is replaced with the single "Photos" thumbnail under the cover, to the right of your contact information.