Can you get pregnant while taking the placebo pills in birth control?

Durr.

Of course you can!
A placebo is a sham medical intervention intended to lead the recipient to believe that the intervention may improve his condition. In one common placebo treatment, a patient is given an inert "sugar pill" and told that the pill may improve his condition. The fact that the pill is inert is withheld from the patient. The intervention may cause the patient to believe that the treatment will change his condition; this belief sometimes causes the patient's condition to change, a phenomenon known as the placebo effect.

So then even if you have sex while on the first day of your placebo pill you cant get pregnant?

Not anymore likely than while taking the other 23 (or 22, depending on your brand/type) pills in the pack.

The first 22 pills prevent the uterus wall from thickening to allow for implantation of the embryo. The sugar, or placebo pills, are there as 'place-holders' and gives a break in the delivery of hormones to allow for menstruation. There is nowhere for embryo (if it has come to that) to implant so it is expelled through your cycle.
That is why it is important to always start your first pack the first day (or week) AFTER your normal period as the uterus wall has not had a chance to thicken at all.
Those 5 or 7 pills are there so you do not get out of the habit of taking a pill everyday. Some types of pills only have the 3 weeks of hormone pills and then you take a week off. I always opted for the 28-day packs as I do not trust myself. lol