With today's Cataclysm announcement, Blizzard clarified how raid lockouts will work in Cataclysm. The raid lockouts are boss-specific, meaning players can kill bosses on either 10- or 25-player during the same week.
This means you could start the week as a 25-player raid and clear as far as possible, then finish up the raid later in the week as a 10-player group to clear the final bosses.
Earlier, the original post had some ambiguous language that could be interpreted as characters being allowed to do 10 AND 25-player raids in the same week, but this has now been cleared up.