SPV helps with some scalability issues, but doesn't solve them all.
From http://bitcoin.stackexchange.com/questions/4649/what-is-an-s...:
> SPV: A Bitcoin implementation that does not verify everything, but instead relies on either connecting to a trusted node, or puts its faith in high difficulty as a proxy for proof of validity. BitCoinJ is an implementation of this mode.
See also:
http://bitcoin.stackexchange.com/questions/11054/understandi...