In the federal system, it depends entirely on the district, but in 90% of them the clerk will give you one "bye" with a good excuse like a cruise coming up. They can always put your name back in, and have it rise quickly to the top next time. Be sure you arrange for an excuse though, some federal district judges are tough on jurors who just ignore a summons. I've even known some to spend some time in jail courtesy of the U.S. Marshal. Very scary.
A broken down old federal prosecutor.
Landlocked in Denver, but cruisin every chance we get.
Polynesia, Carib. '86
Cr. Odyssey, Scandinavia, '91, 30 Day S Pac. 2002
Crystal Harm, Aust., N.Z., '94
Royal Odyssey, AK,'96
Old Cr. Pr. Canal, '97
RCCL, Carib, 1998
Volendam, Car, 2000
Ryndam, 35 day S. Am., Antarctica, '03
Is. Pr., Canal, 2004
Statendam, 34 day China, Japan, AK '06
Cr.Pr., Carib. 08
Eurodam, Atlantic, Med. '10