There are precision voltage references (basically you treat it like a zener diode) that will give you a steady 1.225v, which you feed into one side of a comparator. On the other input, you run your supply voltage through a voltage divider until the tap between the two resistors is also 1.225v at your threshold voltage. I would take the output of the comparator to a latch that, once turned off, doesn't turn back on again. I designed a circuit to do this for a small ATX-like PSU to replace the one that failed in my Popcorn Hour media player. Haven't built it yet, but the sims look good.
If you want to see my test circuit (it monitors +5v and +12v rails for +/- tolerance and issues a Power Good signal), go to Circuit Simulator Applet and let the applet run. Go to File, Import and paste this:
$ 1 5.0E-6 5.934295036739208 50 5.0 50
z 192 144 192 96 1 0.805904783 1.225
g 192 144 192 160 0
O 160 96 160 112 1
w 368 112 320 112 0
r 368 64 368 112 0 10000.0
w 320 16 368 16 0
r 320 16 320 64 0 33000.0
w 432 96 432 128 0
g 320 144 320 160 0
r 320 64 320 112 0 10000.0
r 368 16 368 64 0 29000.0
w 368 64 400 64 0
w 368 416 320 416 0
r 368 368 368 416 0 10000.0
w 320 320 368 320 0
r 320 320 320 368 0 93200.0
w 432 400 432 432 0
g 320 448 320 464 0
r 320 368 320 416 0 10000.0
r 368 320 368 368 0 83600.0
w 368 368 400 368 0
r 192 48 192 96 0 6800.0
172 192 48 192 16 0 6 33.7 40.0 5.0 0.0 0.5 Ref Voltage
w 320 64 352 64 0
w 320 112 320 144 0
w 320 416 320 448 0
w 320 368 352 368 0
172 320 16 288 16 0 6 5.0 5.7 4.3 0.0 0.5 5V
172 320 320 288 320 0 6 12.05 13.5 10.6 0.0 0.5 12V
172 320 240 288 240 0 6 4.800000000000001 20.0 -20.0 0.0 0.5 Supply Voltage
z 352 208 352 160 1 0.805904783 5.0
w 352 64 352 144 0
z 400 208 400 160 1 0.805904783 5.0
w 352 208 400 208 0
w 352 144 352 160 0
w 400 64 400 160 0
w 352 144 432 144 0
w 432 144 432 160 0
w 400 64 432 64 0
r 544 128 544 176 0 1.0
g 400 208 400 224 0
w 240 128 432 128 0
a 432 80 512 80 1 5.0 0.0 1000000.0
a 432 144 512 144 1 5.0 0.0 1000000.0
150 608 96 688 96 0 2 5.0
150 704 112 784 112 0 2 5.0
M 688 96 688 64 0 2.5
c 544 176 544 208 0 1.0E-6 5.003285253570061
r 592 128 592 176 0 150000.0
w 592 176 592 208 0
M 784 112 784 144 0 2.5
w 192 96 240 96 0
a 432 384 512 384 1 5.0 0.0 1000000.0
a 432 448 512 448 1 5.0 0.0 1000000.0
M 688 400 688 368 0 2.5
r 544 432 544 480 0 1.0
r 592 432 592 480 0 150000.0
w 400 368 432 368 0
w 240 432 432 432 0
z 352 512 352 464 1 0.805904783 5.0
z 400 512 400 464 1 0.805904783 5.0
w 352 368 352 448 0
w 352 448 352 464 0
w 352 448 432 448 0
w 432 448 432 464 0
w 400 368 400 464 0
w 352 512 400 512 0
c 544 480 544 512 0 1.0E-6 5.003284999892057
w 592 480 592 512 0
w 544 512 592 512 0
150 608 400 688 400 0 2 5.0
g 592 512 592 528 0
d 512 80 544 80 1 0.805904783
d 512 384 544 384 1 0.805904783
w 544 384 592 384 0
w 544 208 592 208 0
g 592 208 592 224 0
w 512 144 512 112 0
w 544 80 544 128 0
w 592 80 592 128 0
w 544 80 592 80 0
w 512 112 608 112 0
w 592 80 608 80 0
w 688 96 704 96 0
w 704 128 704 176 0
w 240 96 240 128 0
w 240 128 240 432 0
d 320 272 368 272 1 0.805904783
r 368 272 416 272 0 47000.0
c 416 272 416 304 0 1.0E-6 1.8446684058680012
g 416 304 416 320 0
w 320 240 320 272 0
r 448 240 496 240 0 10000.0
w 496 240 496 304 0
w 496 240 512 240 0
r 496 304 448 304 0 33000.0
a 512 256 592 256 0 5.0 0.0 1000000.0
w 592 384 608 384 0
w 512 448 512 416 0
w 512 416 608 416 0
w 544 384 544 432 0
w 592 384 592 432 0
g 400 512 400 528 0
w 848 176 848 400 0
w 160 96 192 96 0
w 704 176 848 176 0
w 688 400 848 400 0
w 416 304 448 304 0
w 320 240 448 240 0
w 416 272 512 272 0
M 592 256 592 288 0 2.5
x 724 116 764 119 0 12 RegOK
w 832 112 832 192 0
w 832 192 832 352 0
w 784 112 832 112 0
x 648 309 677 312 2 12 Shdn
150 704 240 784 240 0 2 0.0
w 704 192 704 224 0
M 784 240 816 240 0 2.5
x 728 245 766 248 0 12 PwrOK
w 624 256 624 288 0
r 768 416 768 464 0 100000.0
g 752 512 752 528 0
d 704 304 704 336 1 0.805904783
f 704 464 752 464 0 1.5
r 752 512 800 512 0 100000.0
r 704 464 704 512 0 100000.0
w 704 512 752 512 0
w 752 480 752 512 0
w 704 464 704 448 0
f 768 464 816 464 1 1.5
w 752 448 752 464 0
w 752 464 768 464 0
w 816 480 816 512 0
w 816 512 800 512 0
w 816 480 832 480 0
w 848 448 816 448 0
w 816 448 816 416 0
w 816 416 768 416 0
d 832 480 832 416 1 0.805904783
w 704 192 832 192 0
M 784 304 816 304 0 2.5
R 848 448 848 416 0 0 40.0 5.0 0.0 0.0 0.5
w 592 256 624 256 0
150 624 304 704 304 0 2 0.0
I 576 320 624 320 0 0.5
w 576 320 576 352 0
w 624 256 704 256 0
w 576 352 832 352 0
w 832 416 832 368 0
w 704 336 704 368 0
w 704 368 704 448 0
I 736 304 784 304 0 0.5
w 736 304 736 368 0
w 704 368 736 368 0
w 736 368 832 368 0
o 2 64 0 34 5.0 9.765625E-5 0 -1
o 80 64 0 35 10.0 9.765625E-5 1 -1
o 74 64 0 35 10.0 1.953125E-4 1 -1
o 109 64 0 35 2.5 9.765625E-5 2 -1
o 114 64 0 35 5.0 9.765625E-5 3 -1
o 118 64 0 34 7.62939453125E-5 9.765625E-5 4 -1
o 141 64 0 34 5.0 9.765625E-5 5 -1
Here's a screen shot. Yours will be simpler, since there's less to monitor. The latch is in the lower right.
