I just got done posting this
there has been to date ONE! Yes one as in a single exploit found on the 360 to enable homebrew.
This exploit started life as the King Kong Exploit but over time all of the entry methods to make use of it have been closed and the final nail was the update to the bootloader to prevent it being used on new or updated units.
So.. Are there people looking?
I am sure there are.
Will they find anything?
Everyone who knows what they are doing say time and time again the 360 is a very very secure system and it was only this one exploit that so far has been found so that should tell you alot.