Scott Prock Scott - Keep the black and white definition of a cap rate in mind. Capitalization rate = Expected Net Operating Income / Market Value of the Asset.
Since actual NOI and Market Value are constantly fluctuating, so will the cap rate. The utility of the cap rate is to exhibit for sellers, buyers and brokers whether the deal is in line with the market at the time of valuation in the buy/sell transaction.
If you have a property with market value of $1,000,000 and NOI of $100,000 (10% cap), will you pay the seller exactly $1,000,000? Maybe. If you can negotiate the sale down to $800,000, great! That however does not in itself change the cap rate. If the market value of said property is still $1,000,000 after it is bought, the buyer still has a 10 cap property that should be able to be sold for $1,000,000.
Using those figures above, if you bought the property for $1,200,000, ouch! The cap rate is still at 10%, and assuming other buyers in your market would only pay $1,000,000 you have additional risk.
If you are analyzing deals involving cap rates, it would be highly advisable to consult with a commercial broker that knows the local market well. The commercial brokers are basically the only source for getting reliable information on the current cap rates are in the market.
Food for thought ... for the cost of a cup of coffee or lunch, you could get some great perspective and education from a commercial broker.