I have updated Rails/Informix to implement DECIMAL support and fix a pair of bugs that prevented the generation of db/schema.rb. I'm happy to say that on its 1.1.0 release, this adapter is mostly complete - indexes is still missing - and robust.
By the way, even though Rails 2.0 is out already, the ActiveRecord's Informix adapter gem was not published. I have tried to bring this issue to Jeremy Kemper's and Ruby on Rails Talk's attention without success yet. So, in the meantime, just donwload and manually install the distribution from Rails/Informix's project site.
Speaking of O/RM adapters, some weeks ago I found another Ruby O/RM called ruby-sequel which looks very nice to me. It has a nice interface that lets you write queries using Ruby and has just implemented an Informix adapter based on Ruby/Informix. You can take a look at some examples here.