Steven Moffat recently recorded an interview with BBC Radio 5, and he reportedly dropped some key details about the Doctor's new companion, as played by Jenna-Lousie Coleman.
He wouldn't confirm the character is called Clara, as has been widely rumored and/or just sort of assumed at this point, but he did say her full name would be released soon. He also said the character would again be a human from contemporary Earth — something paraphrased as being "necessary for audience identification and a ‘jumping on' point for new viewers", which is fair enough really, especially since my main counterargument involves rather loudly declaring,Yes, but what about Turlough!? Anyway, further reports of what he said — we don't have direct quotes yet — suggest "how the companion gets where she is and what that means for the character is what will make her utterly different and fascinating", "her ‘journey' will be shocking and intriguing and the Doctor has never met anyone quite like her before", and finally "her very presence in the TARDIS will change the Doctor and there's something different about her that will have a knock-on effect for the Doctor/companion dynamic." Oh, and she's definitely not linked to any previous characters, so I don't want to hear anything about secret River Song regenerations or what have you. [Doctor Who TV]