I've been fiddling with concepts of minecart computing again, seeing that collisions show much promise. Unfortunately, they also have restrictions - a cart colliding into another one will stop on the spot before the point of collision, so if you try to "read" minecarts as data this way, the debris from one operation tends to impend following operations. Combining collisions with ramps works, but is fairly complicated. I still managed to build an actual "queue" of minecarts where sending a new one in will put it last in line, shifts all present carts one position forward and outputs the first-in-line cart.
Movie:
http://mkv25.net/dfma/movie-2658-powerlessminecartqueueProblems: cart weights must be very similar, or speed loss can lead to misfunctions. The status of the standing carts cannot be examined easily - they have to stand on non-track floor in this version, and pressure plates will only react to carts when built on track. Could be circumvented by running the whole thing at derail speed, so having the "saved locations" as track would still work. Lengthening or shortening the queue promises to be quite tricky, and it'll need a different mechanism to "seed" (i just had carts placed by hand initially).
The benefits are that it needs no power and no signals to advance the queue itself and could be used to shift a block of data by one position _without needing to read the data_, in ~40 steps fixed, adding only three per "bit": exchanging a single cart would take about 40 steps, but advancing a ten-cart queue would take no more than 70.
Each "advancable" location consists of a tile of ordinary floor, surrounded by three track ramps:
#..#
#x▼# ##╗#
#▼▼# #═╚#
#+## Ramps, below
x marks the spot.
Incoming carts come from south, jump over the ramp directly south (because they come from normal floor or go too fast and thus ignore the downward ramp), push the current cart off north (also jumping because speed is still high enough or x itself is once again ordinary floor and not track) and then fall onto the EW ramp, accelerate east, encounter the NE ramp there, accelerate north on it and eventually climb out over the SW ramp onto spot x. The former occupant does the same to the next cart in line, to the north.