I just found out that ESRI has published an official position on open source software on its website at http://esri.com/opensource that sounds quite encouraging. (Thanks to Chaipat Nengcomma's blog post that I could not read but that hinted me to this.)
There is also an interesting podcast where Victoria Kouyoumjian (ESRI IT strategy architect) "discusses ESRI’s position on open source and the importance of open source software in GIS application development" (quoting their own summary).
We already knew that ESRI had been using GDAL for some time, had integrated support for PostGIS in SDE more recently and even sponsored FOSS4G 2007, but to find an official statement of their position is definitely encouraging. I even dare to dream of collaboration between ESRI people and other OSGeo projects in the not too distant future (gotta be optimistic).
I know (for having been through this myself a few years ago) that ESRI has not always had only good words for people who made the choice of open source software for their projects in the past. Let's hope this position is sincere and will propagate across the whole organisation, and that things will really change and lead to a spirit of real collaboration, for the benefit of all the neo-geographers who want the best of both worlds.