Lpc2000

>> 3.  If app can afford 4096+ clk dly post interrupt, enter Power
>Down in RTOS
>> idle loop.  Wakeup condition via external interrupt.  Be sure to
>read the
>> Wakeup Timer section in UM_LPC21XX_LPC22XX_2.pdf
>
>I don't think I can use power down mode, because I need to wake up
>upon new character arrive at the UARTs. Here's my reasoning, please
>correct me if I'm wrong: I think the UARTs are off on power down
>mode, so even if I externally cheat an interrupt at the character's
>start bit, the UART wouldn't wake up fast enough to receive the
>character (at 57600 bauds). It would take at least ~410 \ufffdS to wake up
>with a 10 MHz xtal. One bit at 57600 is ~17\ufffdS.

No clearly your application could not afford the 4096+ clk dly post 
interrupt.
Perhaps a combination of Option 1 and Option 2 is your best best.