It was recently announced that Michelle Ryan will be playing the Doctor's companion for at least one of four specials to be released this year, featuring David Tennant in his final stories as the Doctor.
You may known Ryan from the short-lived and disastrous attempt at remaking the Bionic Woman on NBC last season. Otherwise you would have had to see some recent British TV (such as Merlin) in order to recognize her. I didn't know she was British before now, which explains her bizarrely discomfiting American accent, but not her equally plastic overdone English one.
In short, I strongly disapprove. If she is some mysterious aristocrat (Lady Christina) for just one special, that's more than fine (I mean c'mon, I sat through Kylie Minogue acting in Doctor Who so I can handle just about anything) but if they are seriously considering her as the new constant companion of the 5th series, then Matt Smith is doomed before he begins.
With literally the entire A-list roster of British actors, and perhaps a few adventurous American ones, at their disposal, Steven Moffat and company need to be a bit more original. For instance a male companion who can hold his own and be interesting without needing to rely on sexual tension for dialogue or chemistry. They've always been fans of using past characters, so here's my suggestion; the UNIT solider from Sontaran Strategem who gets brainwashed and later reappears in Turn Left. (Clive Standen.)