Thanks for the comments !
The MultiBit code is all open source so feel free to reuse whatever you like.
Balsamic is very nice if you want to rough up the user interface and give everybody a first impression of your overall concept.
I will also be interested to see what you do with the private keys, as that is the number one question people ask about any wallet (especially web wallets).
Good luck with your project !
Thanks - I really appreciate that the BitCoinJ community is so supportive. I don't need to mock anything up -- it's running right now!
Aside from the BitCoinJ library, I've written all the wallet code myself. I've also added simple flat transaction fee support to the BitCoinJ Wallet (because the Satoshi client strongarms the network by refusing to even relay non-fee transactions that are under 0.01btc, which can even occur inadvertently for change... but maybe that's a rant for another topic...).
MY CLIENT IS NOT A WEB WALLET. Sorry to yell. It's a Java application. Private keys are currently not encrypted on disk (I'm using BitCoinJ-0.5.2
Wallet.saveToFile(..)). I've been following the MultiBit (your?) encrypted private key discussions, and it's very promising. In the future, I will either require a passphrase at startup and save the wallet to an encrypted file stream, or wait for BitCoinJ-supported private key encryption (so passphrase is only needed for sends).
I still have a short list of features before I consider my client 1.0 (or even beta). I tend to be a perfectionist and sit on my projects until absolutely ready, but I like to hear what others have to say, and actually having folks run my software is very motivating.
Although currently I cannot devote much time, I'd love for the next phase of my career to be Bitcoin-centric, so I take this very seriously, and I am very pleased to make the acquaintance of others in the BitCoin(J) community.