Power backup

Hi all, I have a problem of sorts here I'd like to solve. I'm using my arduino to measure an output from another instrument and I power it stright from that (12V). The trouble is that the power supply can be disconnected at any time, but I'd like to keep the last reading in memory. I initially thought about recording avery single measurement in EEPROM or onto SD card, but for different reasons it's not advisable in my situation. So I thought if it's possible to have an another solution: 1. Rechargeble power source (based on capacitors) which would give extra 2-3 seconds of power and let arduino store current reading onto SD card 2. A battery. Have it connected all the time, but when main power is down, save the current value and turn arduino off (is it even possible to do from the sketch?) I'd personally go for #1, but I do not have any idea ho to do that. 555 timer probably? Do you have any ideas on schematics? Please, I need your advise!

http://ruggedcircuits.com/html/circuit__13.html

Thanks! You are a life saver!