As someone who uses DataMapper every day at my job, I would recommend sticking to ActiveRecord unless you are connecting to a legacy database that you don't control the schema of (but I would also consider Sequel if that were the case). Beyond the fact that it is EOL (as a maintainer stated on the mailing list), many gems that need model persistence will support ActiveRecord but it's very rare that they will support DataMapper, so expect to implement support yourself. In my experience if I find a bug I know I will also have to fix it myself due to the low usage / abandonment of DataMapper.
As danmanstx mentioned, the maintainers are mostly focused on Ruby Object Mapper (ROM) now, which used to be DM2. Although some pieces of ROM are feature-complete, it still doesn't have a release date AFAIK (see the roadmap).
If you look at the release-1.2
branch of the relevant DM gems, you can see that it does get minor updates every once in a while. However, there is a backlog of hundreds of issues across the DataMapper gems, many over 2 years old that simply won't be addressed because there just aren't enough DataMapper maintainers (and I don't blame them for wanting to work on something new!).
You're right about the latest RubyGems versions of DM gems having versioning conflicts. If you want to use the latest stable version of DM, I'd advise using the release-1.2
branch of the DM gems you need, e.g.
gem 'dm-core', git: 'git://github.com/datamapper/dm-core.git', branch: 'release-1.2'