To invoke the bootloader from code, use the following code

void ForceBoot (void)
{
unsigned char n;

api_set_BLJB();
api_wr_BSB(0xFF);
n = FCON;
FCON = n | 0x50;
FCON = n | 0xA0;

// perform reset via watchdog
while (1)
{ ; }
}

the functions api_... are from the Atmel example code downloadable from their website

Rob.