My guess is that this is fine if you trust the source or don't mind defaulting. If it's in a block 1/unconfirmed it's 'mine' so it's good to go right? What about 0/unconfirmed, can I still send without problems?
Yes, you send around 0/unconfirmed coins forever from what I've found out in my testing. The only issue is, should those be fake, everyone that the fake coin was sent will have their balance adjusted to subtract the fake coin.
If it's someone you trust (best friend, family, good business partner) then yeah, spend it.
If it's someone you've never done business with, I would wait before spending just in case.
