New in ArcDesktop 10.1 appears to be support for GDAL WMS XML files. If you’re not familiar with the concept, you can check out the documentation here. In 10.0, ArcMap interpreted these GDAL XML files as data files, instead of a raster data source. If you’re still on 10.0, you can hack around the lack of by creating a VRT that references your XML file.
Take note, however, of ArcMap’s GDAL version. ArcMap 10.0 relies on GDAL 1.6, while ArcMap 10.1 uses GDAL 1.8. Many of the WMS minidrivers don’t appear in earlier GDAL versions.
A couple of final notes about GDAL’s WMS support. First off, GDAL is built on top of cURL. One weird fact: cURL can load file-system and FTP URLs just as well as HTTP ones. Finally, for all you windows folk, cURL doesn’t automatically pick up on your HTTP proxy settings. If you want to use Fiddler to see GDAL fetching web content, you’ll have to set an environment variable: http_proxy=http://localhost:8888/.
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So, I said these WMS XML definitions work in ArcMap, but some quick secondary testing shows you can’t create layer package or share these to an ArcGIS Server.