The Xbox 360 hasn't been cracked per se, but the scene has made some pretty good progress since the first firmware hacks were released earlier in the year, the latest of which is the X360. Developed by Team Xecuter (think of them as the elder statesman of the Xbox scene), the X360 allows users to connect the Xbox 360's hard drive and DVD drive (the HITACHI-LG GDR-3120L drive only, right now) to a PC using a USB 2.0 connection. More importantly, it permits dangerously easily DVD drive firmware flashing since you no longer have to mess around with SATA connections. Of course, you wouldn't be flashing the DVD drive to play backups or anything like that. That'd be silly and immoral.
Besides making it easier to flash the DVD drive's firmware, the X360 lets you back up data from the Xbox 360's hard drive to your PC, so in case something happens to your 360's hard drive, you'll always have a copy of the data on hand.
The X360 isn't available just yet, but it will be very shortly. Team Xecuter estimates that it'll cost somewhere in the neighborhood of $44.95 on all the popular unofficial accessory online stores.