Many of the 15,000 students scheduled to receive college diplomas on Long Island starting today and over the next month are graduating with degrees in uncertainty. Theirs is an unhappy economic era of bank failures, corporate bailouts and high unemployment. "It's been a lot of 'we're in a hiring freeze' or 'we're laying people off' - that kind of thing," said Jessica Piperis, 21, of Westchester, who is scheduled to graduate from Stony Brook University with a business and psychology major. "I don't know what I'm going to do."