I've got something similar going on with me. My period is usually regular, but last cycle was a week late and this cycle is something like three weeks late.
From what I understand, one's luteal phase (between ovulation and your period) is generally consistent in length. So what mainly determines the length of a given cycle will be when you ovulate. So if you ovulated late (like when your period was "due") it's possible you could have conceived.
I'm finding it a bit frustrating, since I don't know if I ovulated this cycle at all, nor do I know whether I will ovulate before getting a period, or just skip ovulation all together (an "anovulatory cycle"). Since I'm open to pregnancy, it's not a problem per se, but I'd really like to know when I should test!
If I were you, I'd test because if you did conceive around the 20th when your period was due, you could probably get a positive test by now, since it's been two weeks.
Metis, partner to a peaceful soul, mothering DD born July '11