It's lost permanently if the address is valid but non-existent. If invalid, the client won't even send it out I believe.
Interesting. Won't it just wait until the wallet comes back online. In the case of someone sending to a valid address not yet created would it not suddenly gain that amount when it was created?
I thought it's a bit like a text message. Waits till it turns on then sends it.
If not people need to leave their wallets on all the time and we know that isn't true. So maybe if there was a time limit of X days/weeks/months?
You are getting confused.
You can send to a bitcoin address that is in no one's wallet, which is what Xephan was referring to. In this case, the bitcoins are effectively lost (it would take billions of years to accidentally stumble on the private key related to that bitcoin address).