How many switches? Do you actually need 49 or that an example of what might be possible with a 7x7 array?
Wiring and tips about a matrix here:
http://www.dribin.org/dave/keyboard/one_html/
You may need diodes as mentioned in that post, particularly if you are expecting multiple switch presses at once.
If you only need 16 switches I did a post about that recently:
You could double the number of MCP23017 chips to handle 32 switches. After that I'm not sure whether or not there would be a problem with handling multiple interrupt lines.
Another possibility would be something like the Centipede shield:
http://macetech.com/ (look under Shields).
At the expense of only your I2C pins (A4 and A5) you have access to 64 pins, which you could wire to switches individually. They also have cables and breakout thingummies for connecting to wherever you have your switches.
With that shield you could at least poll the 64 switches - I'm not totally sure about the interrupt side, but polling may be adequate for you. Since each one has its own "bit position" you could readily detect if more than one was pushed at once.
Judging by my copy of the shield, you have access to the interrupt pins on the MCP23017 but you would have to manually run a wire to pins 2 and 3 to actually use them. And even then that would only give interrupt access to 32 of the pins, not 64. Still, that may be enough, or simple polling might do it.