> Right. So it will light up with just 10 mA, just not quite at full 
> brightness. (On the chart I'm looking at, 15 mA looks like it's 
> meaningfully brighter, but any higher than that has negligable effect.)

Yes, every LED datasheet has a brightness against current curve.  Even for 1 
mA or 0.1 mA it will light up, but obviously it won't be very bright!  Note 
that even for 0.1 mA you probably still need a least a volt or so, which is 
why LEDs don't light up at all below a certain voltage.

> Right. So I'm looking at 2.1 V or 2.2 V or thereabouts.
>
> Now looking at <http://focus.ti.com/lit/ug/scyd013b/scyd013b.pdf>, on page 
> 231 (which is actually page 236 of the PDF file), we see that for the 
> 74HC00 I'm looking at using, we have Icc <= 0.02 mA, Iol = -Ioh <= 4 mA, 
> and tPLH = tPHL <= 27 ns. Now, if I actually knew WTF that means...

Dunno, what Icc means here, maybe the current consumption of the device 
itself?

From this page:

http://www.kpsec.freeuk.com/components/74series.htm

The HC series can sink and source up to 4 mA on the output pins if you want 
the signals to still be valid (eg for input to further logic gates).  If you 
are just using them to drive LEDs then apparently up to 20 mA is OK.

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