How to convert USD to Bitcoins
Coinbase.com is probably the easiest. It links to a bank account and debits it via ACH for every purchase but takes approx. 4 business days to get your bitcoins. The price is locked in at the time you make the purchase though, i.e. you purchase 1 bitcoin today for $116. They will begin the debit process. In 4 days you'll have your $116 bitcoin, even if the current price is $120.
They also offer an option to purchase at the bitcoin price after your ACH clears. It all depends on what you want to do.
ETA: Coinbase is going through some growing pains and are often "sold out" of bitcoins, which is why they recently introduced option #2 that I mentioned above.
There is also localbitcoins.com which will allow you to meet up with someone locally to purchase them. I have never used this service and can't really vouch for it.
This post was edited on 4/2 at 7:57 pm