If there are more coins being produced than are being sold bought.
Watch your wording up there. But you've got the general idea. As a finite commodity bitcoin is heavily driven by supply and demand.
There's a thread discussing the supply side here:
https://bitcointalk.org/index.php?topic=130619.0. Note we're at 13% INFLATION currently, which is technically a higher inflation rate than the amount of US currency printed. Will this drop? Yes, and its a very good thing because it means Bitcoins will become more scarce over time.
As for demand, it's really not driven by the number of customers but the actual amount of money willing to purchase at any given time. Your best place to look at this is the order books. *which still must be taken with a grain of salt,
but check out the "depth" tab of this site
http://bitcoin-analytics.com/