Getting mugged once every thirty years means that there's a 3.3% chance that you will get mugged in any given year. According to this data, the Robbery rate in Metropolitan Areas in 2009 was 133 per 100,000, meaning that each individaul stood about .133% chance of getting robbed that year. Note that this data likely includes instances of robbery that we wouldn't think of as mugging, so the actual chance of getting mugged is probably lower.

Edit: apparently Metropolitan Areas include suburbs; To get a better picture of what crime rates are in urban areas, he's the robbery rates for selected big cities (all per 100,000 people): New York 221; LA 317; Chicago 557; Houston 500; Dallas 426; Detroit 661; San Francisco 423; Boston 365. So, somwhat higher than the .133% number I gave earlier, but still well below the numbes that the Grandparent Post implied.