I guess its just that 1-2 years ago I think it would take maybe an hour or 2 to synchronize.. now its at least 10x longer, and im thinkin give it another year or 5 and it might take days? - this could piss some people off that need to do urgent transactions but don't have a pre-synched account handy.
The problem is that it starts at the genesis block and verifies each block one at a time. Old blocks contain very old transactions.
They are planning to have a headers first download scheme. This would download just the block headers (80 bytes every 10 mins) and then work backwards.
Older blocks are inherently less likely to be invalid, since they have been checked over and over.
You could use the network to receive coins and your computer could check that the inputs are known. If check the last 1000 blocks, then it is likely everything (you have seen so far) is valid.
Receiving coins that were last used 2-3 years previously would take longer to verify that the inputs are in the block chain (as your compute would have to go back further) .
Most "active" coins change hands often (by definition), and you are likely to receive one of them.
A transaction could have different confirmation strengths
- all inputs verified to be in the chain?
- how deep is the transaction buried?
A transaction with 6 confirms and where all the inputs are in the chain is pretty likely to be valid.
Even if your computer hasn't verified back far enough to confirm all the inputs, it is possible for the sender to prove that the input exists in the chain (but not that there is no double spend).
