"Unless someone lives under a rock"

This answer is a kind of extended comment on Kosmonaut's answer. I would have said that "unless you live under a rock" tends to be used with situations where the person who lives under the rock is out of touch with current events. Googling for the phrase gives things like

Unless you live under a rock, you’re more than aware about the Federal Election Campaign that’s currently taking place.

Unless you live under a rock, you’ve heard the word blog used sometime in the last couple of years

Unless you live under a rock, you probably have been inundated with news of the upcoming iPhone 3GS.

Unless you live under a rock, you may have heard that starlet Lindsay Lohan is in a little bit of trouble.

etc. It's not used so much with things like "circles are round".

Could the expression in the title be used in a formal setting? If no, what are some alternatives that could be used instead?

I'm not sure how to answer the question you posed since I don't know exactly what "in a formal setting" means.

You want alternatives?

Unless you are totally out of touch with the real world

Unless you never read a newspaper

Unless you have been lost in the Himalayas

etc. I could easily go on all night.

You would probably not use this in a formal setting, but that is probably due to the fact that it would be considered rude. The problem with suggesting a formal alternative is that the sentiment itself is rude.

Unless you live under a rock, you know that circles are round. (=unless you are totally ignorant!)

Some formal alternatives might therefore be things like:

• It should be quite obvious that circles are round.
• It goes without saying that circles are round.
• It is a well-known fact that circles are round.