Lightweight client, bare necessities
In order to correctly show balance and be able to create new transactions, a thin client needs the following information:


Current block number
OutPoint information - hash of the incoming transaction and position of the output relevant to the client.
Signature script, as it needs to be included in a Tx
Amount of Bitcoins send/received in the given transaction
Block number in which the transaction was included (for knowing the amount of confirmations)


(Does anyone have an idea what else could be useful or needed?)