Medicaid could be altered politically, but that's less than $300bn, so even a 50% cut would only dent the deficit by 10%.
That leaves defense (and discretionary spending ($646bn in '11)) as the biggest possible place to cut. A cut of $140bn to defense would be significant, but would still leave the US as the strongest military in the world. Couple that with some significant cuts in discretionary spending and some reforms to Medicare/Medicaid and SS and problems aren't solved, but things look better. Then the reality is that revenue is going to have to increase unless you can get people to vote for giving up their SS and Medicare entirely (or at least in large part). I'm guessing that isn't even sort of going to happen.