So having banged my head against the proverbial wall trying to get the TXB0108 working with this (works fine with other things but NOT for this, unaided), I decided to throw in a BSS138-based I2C-friendly level shifter (both from Adafruit btw). THAT seems to work normally.
As I scoped the signal in and out of the TXB0108, I noticed something: it got very, very noisy as soon as the LED strip was connected to the output. Inputs and outputs both get a high-frequency signal washing them out (on the order of 55.3 MHz, versus the roughly 800 kHz signal I'm sending).
I'm not sure where that's coming from, but I think it's the cause (or the effect of) the TXB switching directions rapidly. In short, it's pretty annoying. It could be that I need more signal conditioning and noise isolation on the output side, or it's just the wrong chip for this job.
So, in short, you will likely have better luck driving the WS2811 via something like a BSS138 FET circuit, or from 3.3v either directly (not terribly safe IMHO), or via a 74LVC245 or another '244/'245 that's got a Vcc of 3.3V (I've not tried it but a few places actually sell it in a DIP... though it'll only output 3.3V at least it isolates the input from the output and for a short data line that'd probably be fine). Using the TXB0108, however, just isn't cutting it.
(If anyone knows of an equivalent FET in a TO92 or TO220 case, I'm all ears... all the better to breadboard with.)
Edit: I stumbled upon a lengthy post on the BSS138 and level shifting here: Is my MOSFET-based bidirectional level shifter insane? - Electrical Engineering Stack Exchange
However, as the BSS138 circuit is working fine for me I'm not inclined to argue with it for this particular application.
Edit2: From what I can tell the 74LVC8T245 may be the perfect chip for 3.3v-5v level shifting when auto-direction switching is not needed (http://www.nxp.com/documents/data_sheet/74LVC_LVCH8T245.pdf). However, it's SMT and that means far more effort if you're breadboarding... that, and I can't find anyone who actually sells it... :-/