Robin Williams and Barbara Bush both undergo heart valve surgery

Actor and comedian Robin Williams underwent aortic valve replacement at the Cleveland Clinic on March 13, 2009. He had canceled his comedy tour earlier in the month when he complained of shortness of breath.
http://news.bbc.co.uk/1/hi/entertainment/7960740.stm
Interestingly, this is the same valve that Barbara Bush had replaced less than two weeks earlier: www.cnn.com/2009/POLITICS/03/04/barbara.bush.surgery/

The aortic valve is one of four heart valves which help direct the flow of blood through the heart. The aortic valve lies between the muscular left ventricle (the side of the heart which pumps blood to the rest of the body) and the aorta (the body’s main artery out of the heart).

The valve may need to be repaired or replaced for a variety of reasons, both congenital (since birth) and acquired. Valves may either too tight (stenosis) or too loose or floppy (regurgitation). Either problem can cause abnormalities in the way the blood flows through the heart and, if severe enough, cause symptoms of shortness of breath, fatigue, congestive heart failure, stroke, or even sudden death. Depending of severity and location, treatment may be done percutaneously (with catheters through the skin) with balloon valvuloplasty (where a ballon is inflated inside the tight valve and splits it open), or with the open heart operation of total valve replacement. In this case, the diseased valve is surgicaly removed and a new one (either a manufactured valve or a natural valve removed from pig) is put in its place.

Overviews
http://my.clevelandclinic.org/heart/disorders/valve/aorticvalvesurgery.aspx
http://www.nlm.nih.gov/medlineplus/heartvalvediseases.html
http://www.nlm.nih.gov/medlineplus/tutorials/heartvalvereplacement/htm/index.htm
http://www.sts.org/sections/patientinformation/valvesurgery/aorticvalve/index.html
http://www.texasheartinstitute.org/HIC/Topics/Cond/valvedis.cfm
http://www.nhlbi.nih.gov/health/dci/Diseases/hvd/hvd_whatis.html

Treatment
http://www.nhlbi.nih.gov/health/dci/Diseases/hvd/hvd_treatments.html
http://www.texasheartinstitute.org/HIC/Topics/Proced/vsurg.cfm
http://clinicaltrials.gov/search/open/condition=