How long ago did you purchase this set? Can you still return it? The fact that the salesman never mentioned it doesn't surprise. I've had a salesperson tell me that 1080i is as high as it's ever going to go. And that was less than 3 months ago. Either he had no clue what he was talking about or he really wanted to sell some televisions. I'm assuming it was the latter.
Regardless of what anyone may say, 1080p is 1080p, and 1080i is still 1080i. No upconversion or downcoversion, fudging, or wobulation can turn water into wine, although you can come pretty close. But I still wouldn't regret purchasing this television. Like MikeSP says, it's a very nice set and you will probably never notice that huge of a difference.