We seem to have several interlocking decisions to make about aeson and dlist in the platform:

Background:

let me know if I got any of this wrong

There is clear agreement to have aeson in the platform.
aeson 0.6.2.1 would require dlist & blaze-builder to be added.
There is a patch to aeson, already in head, that makes aeson use the builder in bytestring, and drop the dependency on blaze-builder. This is good, as a bytestring in GHC 7.8 will make blaze-builder obsolete.
aeson head uses a new Scientific type for arbitrary precision floating point. This will break users of the Number constructor of the Value type. However, it is believed that most users are probably using existing functions to parse to an from standard numeric types when needed, and those continue to work. Unclear how much impact this will have. Also, Scientific could be in it's own package (which would need to be added to the platform), or simply exposed from aeson.
dlist is stable, but could use some love - which was lovingly offered, and we could have a bump which adds some useful typeclass instances.
It was noted that dlist could be replaced by Endo, or explicit use of simple types. There didn't seem to be much support for the idea of altering aeson to do so.

Options for aeson:

skip it in this release
include aeson 0.6.2.1 - requiring both dlist and blaze-builder
include aeson 0.6.2.x, a version with the patch that uses bytestring's builder, and so require only dlist
include aeson 0.7.0.0 - requiring dlist and possibly scientific

Options for dlist, if required by the aeson choice, or because we now like it anyway:

include dlist 0.5
include dlist 0.6, with new typeclass instances added

Options for scientific, if required by the aeson choice:

include Scientific type in aeson
include a new scientific pacakge

Discussion:

I think the best option, if we are ready to embrace aeson, is jump in with both feet: aeson 0.7.0.0, & dlist 0.6. Leaving the issue of is scientific ready to be included as it's own package, or should it just be exported by aeson for now.

Thoughts?

— Mark