I spent part of the weekend toying with Virtual Earth and the somewhat underdocumented VE API. I'm currently looking for a good way to develop a way to replace an existing map interface that, while it's slick-looking and features interactive Flash, the base map is basically a static image- no scale dependent renderers, fixed text height and symbology, and the like. Add to this the current map has been decoupled from any meaningful coordinate system. Having not found any solid Flash-based alternatives that I can put my hands on, and though I like SVG, it introduces the issue of plugin support. Finally, I had been looking at the big three AJAX-oriented offerings.
I see that Schuyler Erle has also been evaluating these as well- with his Google Maps Hacks background, it's interesting to see his perspective. The things that I am still grappling with most are basic - I'd like to overlay my own polygon layer, and have the polygon symbology linked to a dynamic web service that updates once a minute.
Now I have seen some interesting things on the Google side - VGMap is one attempt to accomplish it, but the map behavior and quirks are still kind of rough...
But by far the most interesting thing I've come across is the Virtual Earth Shapefile Reader - it appears to use a web service that converts the .shp and sends it out as XML. The application can then call the web service and bring in the ESRI shapefile on the fly. It's close, but not quite, to what I need. But the feel of the map ends up quite nice.
If I'm missing any other offerings out there, or if anyone has suggestions, definitely drop me a line.