According to comedy website Chortle Lee Evans will appearing in the Easter Doctor Who special. The story (which contains very slightly spoilerish plot details) also states that Michelle Ryan has been cast in the special (as previously rumoured). This is also reported in The London Paper . So it's still looking more likely that she's playing the Doctor's companion for this one time only, and will not be the permanent sidekick for Matt Smith. Then again, look what happened with Donna Noble.
Evans may be best known as a comedian (well, judging on your point of view) but he has had numerous acting roles in films such as There’s Something About Mary, Mousehunt and The Fifth Element. So no need for Ken Dodd-induced panic attacks yet.