From the comments about Google Maps Hackers:
It’s all moot. Hardly anyone, including the GIS professionals, attaches metadata anyways. Even when you do, nobody else knows what do with it.
Actually ESRI does include metadata with their ArcWeb Services datasets. Take a look at the U.S Street Map Service metadata page. This information is available for every ArcMap service. But it isn’t just ESRI. Geodata.gov has extensive metadata as well as other providers of data (when you get satellite imagery from DigitalGlobe, they give it to you).
Before the API was released, it really wasn’t a big deal to know how up to date the information is on these web mapping services, but now that they have opened up the API, publishers need to know how relevant the information is before investing time in Google Maps. Do the hackers care? Maybe not, but if they expect professionals to ever develop using the Google Maps API, this will have to change. ESRI’s ArcWeb does offer this and I suspect when AWS 2005 is released will reap the benefits of having their metadata available for every data layer they offer.