Even after the deadline, you can clarify things in comments in response to questions from mentors. And of course, you can still discuss the project with us on IRC and the mailing lists, and participate in the Mono community. So it's best to make sure you get the application in, and don't leave it till the very last second.
If you're a student still looking for ideas, take a look at the Project Ideas list (though you're welcome to come up with ideas of your own). Please also read the application template before applying. It's a good idea to discuss the project with us on IRC or mailing lists first, though now it's so close to the deadline, you should err on the side of submitting early and discussing afterwards.
Mentors, please make sure you're signed up so you can start reviewing applications.
Everyone who's interested in Summer of Code, please join the community on Mono IRC, including the Mono SoC channel, #monosoc, and the Mono mailing lists. And please spread the word to anyone you know who might be interested - there's still time to write an application!
If you have any comments or questions, please email Michael Hutchinson.