For the record, this functionality did NOT exist when I first posted this question. I asked on the mailing list and was informed by Milan Broz that discard passthrough for dm-crypt targets still had not been implemented. Milan indicated that he intended to implement this at some point, but did not state when.
After digging around in dm-crypt.c and related files for a while, it appeared that TRIM support would be fairly trivial to add. I wrote the list asking for their general advisement on the task before I jumped in. The next day, Milan submitted a patch that implements TRIM pass-through on dm-crypt and this has been staged into linux-next. It should be included in the Linux 3.1 kernel.
TRIM passthrough must be enabled explicitly. There are potential security issues in using TRIM on a dm-crypt'd device, because TRIM sends information about block usage to the firmware on the device which then marks recently freed areas as usable (as I understand it, anyway). This means that an interested party can derive information about patterns of disk usage. For instance, it was postulated that an attacker could almost definitely learn what filesystem was in use on the encrypted medium through this data. An attacker may also be able to learn more useful information, like that you had a lot of big files saved until recently (big contiguous TRIM'd blocks).
Please see these threads for reference:
tl;dr: TRIM passthrough support for dm-crypt targets will exist in Linux >= 3.1 but must be enabled manually via cryptsetup and/or dmsetup due to data leakage that may allow profiling based on disk usage patterns.