Carter was originally signed to the Patriots' practice squad on September 7, 2010. He spent most of the season there, and the Patriots eventually had to sign him to the active roster in December to assure them that no other team could snap him. He played in just two games and recorded just one tackle, but he had won several "practice player of the week" awards throughout the season.
That is what makes Carter's release somewhat surprising. While he probably was a long shot to make the 53 man regular season roster, he was young (25) and cheap, meaning that he made sense for training camp competition for a team that needs to fill out their roster. Losing Carter will not be a big deal in the big picture, but the release comes as somewhat strange to me. We will see if there's more behind this story.
While Carter has been informed of his release, teams won't be able to officially let go of players until Thursday.