We are starting to see properties "not appraising out" lately. That's an early indicator that prices may be increasing, at least in submarkets or areas where supply is limited. What we mean by that term is that the appraisal number is lower than the amount on the sales contract. If the buyer needs a mortgage, it's an issue, because he/she may not be able to borrow the full amount that they expected. In other cases, where the buyer is paying cash, it is not uncommon for the buyer to put in a clause that the sales price cannot exceed the appraised value.
There are two ways to look at this issue, as with so many things. One is that the buyer should be willing to pay what a property is worth to him/her, regardless of the appraisal. The other side of the coin is that the practical value of a property--what it would resell for, and what you could borrow against it--is dependent upon the appraisals.
Appraisals tend to lag the market, because an appraiser can only use comps from within a narrow range of closing dates, and within a very small area around the subject property. It can be very hard sometimes to find good comps, and adjustments must be made from properties that might not be exactly the same in quality or type. Since the comps come from recent sales, those prices could be lower, in an increasing market, than the sales prices on contracts that haven't yet closed.
What advice do I have after all of this discussion? Appraisal is as much an art as it is a science, and both buyers and sellers should be reasonably skeptical about exactitude. Given the restrictions on bank lending these days, it is harder for a bank to take the same attitude, so just be prepared to make an independent decision in any given instance.