well you can't make a per second statement, since, in theory, their could be thousands of blocks in one second.
and because all of the transactions in one block get processed at once, it's hard to say how many transactions per second there are.
I guess you are looking for something like this:
"How many transaction fit in one block ?" and "Is there a maximum of blocks per second in the network ?"
Isn't there an average number if blocks generated per unit time that will roughly remain constant over long enough time periods? (as difficulty adjusts to compensate for changes in network computing power)
This answer, combined with
"How many transactions can fit in one block ?"
should get us close to the answer we're looking for.
edit: according to the FAQ
The average rate of block creation in the whole system is 6 / hour.
So the rate is 6*(max transactions per block) per hour.