Oooh, I'll have to look at that later too, thanks for the link!
The mirror recognition tests are interesting - there are some species that if you alter their appearance while they are asleep and unknowing, when they wake up they will look in the mirror for a long time, touch and themself and the altered area curiously as if they know it is them they are looking at. Chimpanzees seem to think that at first it is another chimp, but then get very curious - I've seen video of Kanzi the bonobo with a flashlight and a handheld mirror inspecting as far as they can down their throat!

It's hard to know what to make of it though - is it really self awareness? I was spending some time with a little 2 year old the other week, and she was on my lap and we were playing with my phone. The picture setting was on, so we could see ourselves on the screen. I was urging her to make funny faces with me to take pictures - like stick out our tongue, etc - and she found it entrancing and funny, and after a bit of encouraging made the faces - but I'm not sure she connected that what we were watching was us in real-time.
For example, after we took a few photos, she then requested "Take picture of [her name] in the park!"
And I laughed and said I can't, cause she's not in the park. Puzzled, she than asked, "Take picture of [her name] in the garden!"
So I'm not quite sure she totally got it... though I'm also not sure she'd pass the mirror recognition test either. Should have done some experiments....
As for the connection between this ability to think about thoughts and its connection to language, this is where I get really interested:
-Language and theory of mind seem to bootstrap each others' development in children. For example, language is necessary to appreciate certain mental states of others. Traits such as gaze following and joint attention seem to be important for both language learning and theory of mind development.
-In Astington and Jenkins (1999), they studied 60 children at three different points over 7 months, to assess their theory of mind ability and language ability. They found earlier language ablility predicted later language test performance. They also divided the contributions towards semantic and syntactic ability and found it was specifically syntactic ability that predicted the theory of mind score.
Here's a .pdf document that summarizes the research well and gives a good literature review on the subject of theory of mind and language!
http://cls.psu.edu/pubs/pubs/Miller_06_offprint.pdf