ESRI has been talking about this for months (and haphazardly blogged about on the ArcGIS Online blog last week), but they’ve now got a post up on their ArcGIS Server Blog outlining the changes coming to ESRI’s ArcGIS Online web services. (Side note, wouldn’t be nice if ESRI sort of planned their blog posts better for a common message?)

Since the release of ArcGIS Online three years ago, the 2D services have used the WGS 1984 geographic coordinate system and a 512 x 512 pixel tile size. Google and Bing, in contrast, use a modified Mercator projection and a 256 x 256 tile size. The scale sets used by both tiling schemes are similar, but not equivalent.

Make sure you read the whole ArcGIS Server blog post to understand what you have to do and when. From the ArcGIS Online blog:

The existing services in the?ArcGIS Online tiling scheme will remain available for at least six months and, depending on demand, may remain available longer. Although the services will remain available, the content in these services will no longer be updated.