There is no way of knowing how much USD 1 Bitcoin is "worth" at the moment. It is it's own currency, so there is no fixed value of USD <--> BTC that you could apply and everything else would just mean querying a centralized database/source.
If you need something like this, maybe take a look at BitPay.
Yes, and is there a good reason to not do so?
I know that Bitcoinspinner does this, but only on the homepage but not on the sendpage.
I can see the value of my total coins on that page.
So it should be possible.
But when I need to send a value of X in a currency I need to bring my calculator.