I think I'm getting confused here. At first you talked about a lightweight client connected to a trusted node back at the office. Now it's untrusted.
A user of an independent, lightweight client can either have one trusted full client he intentionally connects to, such as his full client on his home PC; or can connect to a number of random untrusted peers. The end result is the same, since your full client at home is ultimately connected to numerous untrusted peers itself. The trust comes from polling multiple sources and comparing the results until it becomes statisticly remote that you have been spoofed. The point of forcing the mobile client on your smartphone, lightweight or not, to connect to your single trusted client at home is so that you can move the resource consumption issue to the full home client and off of your wireless data plan. Likewise, the lightweight client that only keeps the block headers locally does so to reduce local storage consumption; but for now, this really isn't an issue on the home PC.
Also, I said it'd be possible to do transactions without internet access for buyers only - I agree there are no situations in which sellers would want to use BitCoin without internet access. And mostly both sides would have it so there's really no issue here.
Perhaps I misunderstood.
Client-mode can still be useful even for sellers, consider the case of you paying a restaurant bill and then asking your friends to contribute their share by tapping their phones to yours.
Your freinds presumedly already have your trust, so there isn't really a need for the lightweight client to be able to verify that independently in real time. Likewise for moving funds from your full home client to your mobile client, as you can presumedly trust yourself. The lightweight clients still need to have copies of the blocks that contain your coins, so that they can properly create a spend transaction; so if two light cleints are connected ad-hoc so that one person can send the other funds, a copy of the referancing blocks can move in the same manner, but you had better be able to trust the person giving you the transaction.
On lightweight clients. You could build a system in which only the wallet was hosted locally and it connected to a specially built server. The server would be given the output addresses, a list of every public key in the wallet and create a partially complete transaction which would be sent to the client. The client would sign it and send it back to the server, which would then relay it across the network for you. The client relies on the server to tell it the current balance.
You could do this, but then the light client wouldn't really be independent, and it's practically the same as using a remote control client to tell your full client at home to do the transaction.
That model is lower resource usage than even the 'simplified payment verification' model and might be worth investigating if client-mode is still too resource intensive for smartphones. The big downside is you lose all privacy. To build transactions for you the server needs to know all your public keys and thus, your entire balance and naturally gets to correlate them all together as a result. It can't spend your coins but it can track you pretty well.
The privacy issue isn't an issue if you own the full client also, but the point of a lightweight client is to be able to transact sans Internet.
