AVR RISC processor @ 13MHz, 1Mb flash (512x16), 32kb on chip RAM (no RAM chips on PCB), 8kb EEPROM, 33x101 pixels graphic LCD. Pretty much everything is known: including display, keyboard, beeper, sim card comunication; but not radio parts. Here. Nice libraries exist, and it is all enough to replace default firmware with arkanoid game.
Z80 processor @ 13MHz, 1Mb flash, 32kb RAM, 8kb EEPROM. Nicely reverse-engineered, too; has similar hardware to A1018. Here.
You can get pictures of internals of many phones at inside-gsm