I recently had my oil changed at Wal-Mart and they left my oil cap off. I did not realize this until I drove to Arizona from Colorado towing a Honda Prelude. Granted my truck has 170,000 but I was the original owner and never had to do any major engine repair. I took my truck to the shop because it was acting funny and they replaced the PCV Valve, plugs, coil wires, and distributor cap. After all this I was still having trouble starting my truck. After a few weeks of them not finding the problem my engine light came on, cylinder 2 misfire. They are telling me that I need a complete engine rebuild, is there any chance that this is from driving my truck without the oil cap?

I did that too but for only half as many miles. I'm not an expert but unless it were in very dirty, dusty conditions, or if some freak object bounced into the opening, I don't think it would cause much damage.

I would say that without the oil cap in place the worst that could happen would be some oil could blow out. Did you see any signs or get a low oil warning light?
There is often some type of baffle inside the valve cover that is intended to prevent anything too large from entering the crankcase, however, I don't know what you're driving so I can't say for certain...
Any starting problem would likely turn out to be either battery/starter issue, or fuel delivery related concern. A misfire on a single cylinder should be readily diagnosed by a competent tech with the vehicle right in front of him or her.

What year, make and model are your truck? When they replaced the plugs, coil, wires & cap, did they also replace the rotor? If you're driving a GM, you may need to stick with OEM cap & rotor; I've had a few burn out prematurely. I can't believe you'd need an entire rebuild due to a misfire; I would get a 2nd opinion.

To answer your initial question, no, I don't believe this is due to your oil cap being off. As Rich said, you may have lost a bit of oil but that doesn't affect the firing in the cylinders.

Get a second opion that's for sure,get a compression test done also,that will tell you for sure if the # 2 cyl is shot,could be a burnt vale of rings on the cyl.They will have to do a dry and wet compression test to deterim this.As fot the oil cap not being causeing the problem,most unlikely.You mite have got a bit of dust in the crank case or lost a bit of oil.You could tell yourself if you lost oil out of the filler,oil would be all over the place, most vehicals have a shroud under the filler cap that won't allow oil to fly out of the hole,good luck.