I thought this post from Kevin Drum summed it up fairly well. The problem is medical cost inflation. Social Security is very close to fully funded through the next 75 years as long as Congress treats the trust fund treasury bonds the way it treats regular treasury bonds. Discretionary spending can be(and is) changed year to year. The real problem is that medicare/medicaid costs are rising much faster than revenues.

http://motherjones.com/kevin-drum/2010/11/deficit-commission-serious

In particular, look at the CBO graph.