Is that something you'd like to see in a Bitcoin wallet ?
That would help lessen the fears that exchange rate risk would cause the value to drop significantly.
Would Strongcoin need to hold the funds for the USD wallet though? This brings centralization, like what Mt. Gox offers with their mobile wallet or Bit-Pay's client does, I believe.
I wonder though if the same concept employed for StrongCoin's BTC wallet could be employed for USDs, but instead of storing Bitcoin private keys it stores redeemable codes from an exchange instead.
Then to spend, the redeemable code is redeemed at the exchange, and a change transaction for any unspent amount is returned as a new redeemable code stored in the user's wallet in the browser.
This way, the funds are never held by Strongcoin. If access to the users's exchange account credentials were available, the code could even do thet transaction directly without even touching Strongcoin (i.e., using the API for the redeemable codes). Though I think for a newbie handling this without a second set of credentials (to the exchange) would make it even more useful, thus some intermediary for spending funds from the redeemable codes would enable this and would work nicely it seems.
Is this anything along the lines of what you were considering (or would consider)?