This is Derpy:
As we know, in the MLP:FiM universe, cutie marks indicate a pony's special talent or individualizing feature.
Derpy's is a bunch of bubbles.
What does this cutie mark mean? Or, more fully, what is Derpy's special talent?
This may be too much of a general-reference sort of question, but still.
Out-of-universe, she was originally just a background pony/pegasus and the bubbles mean nothing.
In-universe, Derpy is cheerful yet clumsy, so the bubbles are a reference to being a bit of a bubblehead.
(Note: Despite the gravatar I've used since Christmas 2011, I am not a "brony," but I've watched all of S1 of MLP and most of the MLP movies from the generation just before FIM with my daughter.)
Her "Bubbles" cutie mark is because of Faust's liking of The Powerpuff girls, and one of the Powerpuff girls, called "Bubbles" often acts silly (or Derpy) at times.
In a twit, Lauren Faust confirmed that Derpy's cutie mark was in fact helium bubbles.
Regarding pegasus helium farts: I think we finally know what Derpy's cutie mark is.
That being said, I suspect she might have been joking.
I have heard about derpys cutie mark having to do with bubble wrap because she supposedly deliveres mail. So i guess she wraps it in bubble wrap. The power puff girl thing is most likely though
Someone once said that it may have something to do with diving/swimming, due to some evidence related to a book in the show (I don't remember the details)
Derpy is bright, cheerful, and bubbly, so I think it would be appropriate to say it is a bunch of bubbles, because derpy is so full of life and never looks at the bad side of things!
This answer is tricky, but I think the bubbles refer to the girl Bubbles from the PowerPuff Girls because Bubbles looks like her sisters but is completely clueless, just like derpy.