None of those low cost optocouplers are designed for high speed.  Look up high speed photocouplers in a disti catalog and you'll get sticker shock. The 4N35 datasheet I'm looking at (vishay) says 110 kHz cutoff. Based on Ton and Toff times, you should be able to gt a bit more than that (as you seem to be doing). One thing you can do to coax more speed is decrease the collector current.  I'd shoot for 2mA or maybe even 1 mA. You're north of 30 mA.  Definitely lose the coupling cap.  In fact, I'd lose everything except the pullup resistor (and the emitter resistor, of course).  Try not to load the output - I'd feed straight into the Schmidt trigger.

you can use the analog pins for digital - the digital pins run from 0 to 23.  The analog pins A0 - A11 run from 22 to 11 (i.e. they overlap in reverse order).  See the pinout card they ship with the teensy.  So just use 0-23 and don't worry.

I also have the open logic sniffer and like it more than the other posters.  The fact that it costs $50 (don't buy from Sparkfun for $70) is a factor in that.  It's pretty darn useful, though.  The client code could be better but it does what it's supposed to do.  I don't mind that it downloads the data when the capture is done.  It's pretty quick for that. 

On the issue of echos on the unused lines.  This is a problem because?  Like you don't like seeing them on the display?   They are pretty innocuous.  I have a stripped piece of wire with a clip on the end to which I attach all the unused probes and then clip to ground.  I like it more because it keeps my probes from flopping around, though, and often don't bother with ground.

I think it's a great LA for learning about LAs.

I know this is late but maybe for other reading the thread...

I got the open logic sniffer which is open source hw built for SUMP.  $50 for the board from Seeed ($70 from sparkfun). Probes were a few bucks more and I found a reasonable case to hold it for <10. 16 channels.  The current incarnation claims capture rates up to 200 mhz.  The java based client is ok though suffering through getting the right version of java on my machine was an exercise in patience.

Here's the main page - http://dangerousprototypes.com/2010/02/25/prototype-open-logic-sniffer-logic-analyzer-2/

I've had it a month and so far I've used it for some mundane stuff - debugging i2c, spi, lots timing stuff - it worked perfectly.  Given that my all in cost is about $75, I am way happy with it.