This is indeed a very usefull feature / #5534

Several days ago I created a new task to request exactly this, because I didn't find anything related to that (until now, probably overlooked it)

As I said in my request Allow creation of tickets, tasks, etc by mail Allow creation of tickets, tasks, etc by mail, products like Eventum (eventum.mysql.org) or Request Tracker (www.bestpractical.com/rt) already have this feature

Most of the time, customers report issues by mail and being able to add those mails directly into the sytem can be very usefull (certainly it the system allows replying on those items)

I don't think it's that difficult or that much work.
Just use the php smtp functions to read incoming mails and add them to a queue (a new table in the database ?)
Then create issues or comments based on those items.