I just downloaded Bitcoin 0.5 and started downloading the blockchain from scratch. The process was rather fast early on but slowed down noticeably later. That could of course be related to network conditions (or I could be mistaken altogether), but it got me thinking if the cost of block validation (which apparently is the driving factor in determining how long importing new blocks takes) somehow depends on the length of the chain before it?
Validating hashes and signatures for transactions is the most costly process. So the amount of time it takes for CPU to verify a block depends on number of transactions in the block.
Number of transactions per block tends to increase as bitcoin economy grows, therefore it takes longer to verify more recent blocks.