The 10% figure comes from (*drumroll*) Alfred Kinsey... Or rather, it originated in his book, "Sexual Behavior in the Human Male".
Does this mean that 10% of the population is homosexual? No, it means that, of the 5300 white males polled by Kinsey and his peers, 10% answered that they were predominately homosexual.
Different polls and different sources have their own figures, however. A 1993 Janus Report estimated that 9% of men, and 5% of women, had more than "occasional" homosexual relationships. The Family Research Report says "around 2-3% of men, and 2% of women, are homosexual or bisexual." The National Gay and Lesbian Task Force estimates 3-8% of both sexes.
As someone pointed out in the comments, it's hard to establish exactly what "homosexual" actually means. If a man has one homosexual experienced during his lifetime, was he gay? Was he bisexual? Was he straight? Whatever category you'd place that man into, Kinsey's reports indicate that 37% of men would fall into it, too.
In short: There's no definite, conclusive answer.