The problem is you still have to wait to check if the transaction actually took place.
I am not sure what problem you are referring to. Please elaborate.
Maybe Findeton refers to a problem of sending couple transactions in quick succession? One needs to wait for each transaction to be picked up in a block then retrieve the TX outs again from server-side blockchain?
Yeah, that is a problem with only having one private key. I tripped up on this with StrongCoin too (although a workaround was implemented of allowing a spend to multiple addresses--this makes the problem much less severe.) Allowing multiple spends at once will help BitcoinSpinner too, but considering the simple design, it should probably be something the user has to ask for specifically. Also... while a little confusing at first, it might help to list the change itself as also being "coins on the way" ("change on the way?"), with the balance of the mini-wallet being 0.00 for a while after a spend.