I can't find absolute confirmation on this but according to a couple of travelers on another site, the authorities were refusing entry to the Colosseum to tourists with backpacks/daypacks yesterday. As there is no sort of cloakroom/luggage storage there, this obviously created some problems.
I'll see if I can find anything official but I thought it worth the mention as packs of all sizes are fairly common among sightseers. It could have been a one-day thing or it could be the rule from here on out. Any members in Rome with current information?
Even if just a temporary situation, it may be a good example of the sorts of unexpected security measures travelers may experience these days. Grin and bear it!