Squareup looks like it might be pretty awesome, just in general. At first glance at least.
Wrt Bitcoin, it could be excellent for face-to-face transactions. Swipe the card, wait for the money to clear, use your phone to send the bitcoins. Though, I can't imagine how there would be a significant amount of business for someone selling bitcoin in the streets like that.
Since a chargeback against a credit card transaction is easy, I don't know that this would be an option for selling bitcoin "in the streets", though I suppose for friends and family it might be suitable. 2.75%. No different than using PayPal, except that PayPal has the "personal / gift" option where no fees are incurred by the recipient. AmEx's Serve is another credit-card + ACH based payment network similar to PayPal, and for now (through August, 2011) they charge no fees to either the sender or the recipient.
Again, if you trust that you won't get charged back, these all work fine for person-to-person payment methods.
http://squareup.com http://gopayment.com http://serve.com http://dwolla.comThere's a good list of options being built here:
http://bitcointalk.org/index.php?topic=7069.0Exchanging in and out of bitcoins for each transaction certainly gets to be expensive though.