Switch’s answer is correct for the LHTR, and many other revised rulesets such as Caspian Sub’s. It is not correct for the out of the box rules however. The official Avalon Hill/WOC FAQ spells it out pretty clearly:
@http://www.wizards.com/default.asp?x=ah/faqs/axisrevised:
Q-IPC loss from strategic bombing is limited to the territory’s income value. Is that per bomber or per turn?
A-Per bomber.
So in other words, if the US with three Bombers bombs Germany and rolls 4,6, and 5 it does 15 IPCs of damage to Germany, since none of the bombers invidually exceeded the territory value of 10.
Germany bombing the Causus with two bombers with rolls of 2 and 5 does 6 IPCs damage to Russia, since each bomber can only inflict a maximum of 4.  So the roll of 5 is downgraded to 4.
I’m also pretty sure that in the out of the box rules, Rocket attacks and Strategic Bombing attacks are resolved separately. So, for instance, Germany can launch a Rocket attack vs. the Causus to inflict 1-4 IPC of damage and do a Strategic Bombing run on the Causus to inflict 1-4 IPC of damage per bomber all in the same turn. Switch’s comment that the two are combined is correct for LHTR though.