Who knows. Salesperson can play tricky on you sometimes. It's just a heads up and at the same time, it does make sense.
edit: this is what I found on Best Buy's website. Although this aimed towards for preordering games and what not, I am pretty sure that the same rules will apply for cellphones:
"Q: Can you ensure the price listed at the time the pre-order is placed will remain the same?
A: Yes and no. When a price is assigned to a pre-ordered item, we are reasonably certain it will remain at that price. However, from time to time — given how far in advance we make items available to our customers — a price change will occur. If this happens, you will always receive the lower of the two prices. For example, if you order an item for $49.99 and the pre-order price is later dropped to $39.99, you will be charged only $39.99. If you ordered the item at $49.99 and the price rises to $59.99, you still pay only $49.99. When you place a pre-order, you are assured our lowest pre-release price."
So this probably means that even if it raised to 299.9 while in the first place was 249.99 when preordering, People who preordered the Thunderbolt will get it for 249.99 (lowest price which was stated at the end of their sentence).
You may correct me if I am wrong, but just trying to help you guys and clarify things up.
If Best Buy fudge us over, no comment. Done as much research possible on their "policy".