SDA / SCL is 20 / 21 on the mega.
Just soldered a header to a 16x2 lcd and used that on your phi-1 shield.
For the mega the A and B button are 54 and 55 respectively. You might want to add some compiler directives to have this work for mega users.
Maybe something like this would do:
#ifdef ATmega1280
#define btn_b 54
#define btn_a 55
#endif
#ifdef ATmega2560
#define btn_b 54
#define btn_a 55
#ifdef ATmega328
#define btn_b 14
#define btn_a 55
#endif
The only thing I have against the RJ connectors is that I'm afraid someone will fry his arduino by connecting a telephone/ISDN/ethernet device.
The good thing is of course availability of cheap cables connectors and crimping tools.
I like your new design especially the two leds 