Please read http://www.bitcoin.org/bitcoin.pdf . Block size is a solved problem. The solution just hasn't been implemented in the reference client yet since it's not a big problem yet. In the future, the regular user will only keep historical blocks with transactions involving their addresses, plus probably the X most recent blocks. Poof, each irrelevant (to this user) block is now 40 bytes.
There ARE solutions but what you described isn't a solution and is horribly insecure.
You either need the complete block chain (possibly pruned) or to rely on a trusted source who has the complete block chain.
If you are relying on a partial block chain for validation you can be easily spoofed. How do you know that an address sending you coins is valid? How do you know that an address has the right number of coins? If you "see" on the network a re-organization how do you know those blocks are valid?
A complete block chain is required so each coin can be traced back to genesis. The client is checking the "chain of custody" to ensure the address does have the balance it says it has. That being said an individual user doesn't need to have a complete copy as he can rely on a third party (block chain server, lite client, ewallet, etc) but a partial copy of the block chain is a good way to get robbed.
. 
