I believe proprietary datatype sniffers included in tool shed repositories are loading as expected - at least I cannot reproduce the behavior you are seeing. The datatypes_conf.xml file included in the latest revision of the gmap repository on the main Galaxy tool shed looks like this:
Here is the snippet of my paster log when I install the gmap tool shed repository - notice that the tools, datatypes and sniffers are all loaded. I'm installing it from a local tool shed, but I downloaded the latest version from the main Galaxy tool shed and uploaded it with no changes to my local tool shed for testing.
galaxy.web.controllers.admin_toolshed DEBUG 2012-02-06 10:50:33,686 Loading new tool panel section: gmap
galaxy.util.shed_util DEBUG 2012-02-06 10:50:33,687 Installing repository 'gmap'
destination directory: gmap
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 10 changes to 10 files
updating to branch default
10 files updated, 0 files merged, 0 files removed, 0 files unresolved
galaxy.util.shed_util DEBUG 2012-02-06 10:50:34,062 Updating cloned repository to revision "dbcccd1e4dfd"
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
docutils WARNING 2012-02-06 10:50:34,388 <string>:10: (WARNING/2) Explicit markup ends without a blank line; unexpected unindent.
galaxy.util.shed_util DEBUG 2012-02-06 10:50:34,437 Adding new row (or updating an existing row) for repository 'gmap' in the tool_shed_repository table.
docutils WARNING 2012-02-06 10:50:34,533 <string>:10: (WARNING/2) Explicit markup ends without a blank line; unexpected unindent.
docutils WARNING 2012-02-06 10:50:34,657 <string>:10: (WARNING/2) Explicit markup ends without a blank line; unexpected unindent.
galaxy.tools DEBUG 2012-02-06 10:50:34,718 Reloading section: gmap
galaxy.tools DEBUG 2012-02-06 10:50:34,769 Loaded tool id: gvk.bx.psu.edu:9009/repos/test/gmap/gmap/2.0.1, version: 2.0.1.
galaxy.tools DEBUG 2012-02-06 10:50:34,813 Loaded tool id: gvk.bx.psu.edu:9009/repos/test/gmap/gmap_build/2.0.0, version: 2.0.0.
docutils WARNING 2012-02-06 10:50:34,846 <string>:10: (WARNING/2) Explicit markup ends without a blank line; unexpected unindent.
galaxy.tools DEBUG 2012-02-06 10:50:34,877 Loaded tool id: gvk.bx.psu.edu:9009/repos/test/gmap/gsnap/2.0.1, version: 2.0.1.
galaxy.tools DEBUG 2012-02-06 10:50:34,911 Loaded tool id: gvk.bx.psu.edu:9009/repos/test/gmap/gmap_iit_store/2.0.0, version: 2.0.0.
galaxy.tools DEBUG 2012-02-06 10:50:34,962 Loaded tool id: gvk.bx.psu.edu:9009/repos/test/gmap/gmap_snpindex/2.0.0, version: 2.0.0.
galaxy.datatypes.registry DEBUG 2012-02-06 10:50:35,147 Loading datatypes from /Users/gvk/workspaces_2008/shed_tools/gvk.bx.psu.edu/repos/test/gmap/dbcccd1e4dfd/gmap/gmap-93911bac43da/tool-data/datatypes_conf.xml
galaxy.datatypes.registry DEBUG 2012-02-06 10:50:35,159 Loaded sniffer for datatype: galaxy.datatypes.gmap:IntervalAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:50:35,160 Loaded sniffer for datatype: galaxy.datatypes.gmap:SpliceSiteAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:50:35,161 Loaded sniffer for datatype: galaxy.datatypes.gmap:IntronAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:50:35,161 Loaded sniffer for datatype: galaxy.datatypes.gmap:SNPAnnotation
If I stop and restart my Galaxy server after I've installed the gmap repository, everything loads correctly as well:
galaxy.datatypes.registry DEBUG 2012-02-06 10:57:10,713 Loading datatypes from ../shed_tools/gvk.bx.psu.edu/repos/test/gmap/dbcccd1e4dfd/gmap/gmap-93911bac43da/tool-data/datatypes_conf.xml
galaxy.datatypes.registry DEBUG 2012-02-06 10:57:10,715 Loaded sniffer for datatype: galaxy.datatypes.gmap:IntervalAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:57:10,715 Loaded sniffer for datatype: galaxy.datatypes.gmap:SpliceSiteAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:57:10,716 Loaded sniffer for datatype: galaxy.datatypes.gmap:IntronAnnotation
galaxy.datatypes.registry DEBUG 2012-02-06 10:57:10,716 Loaded sniffer for datatype: galaxy.datatypes.gmap:SNPAnnotation
Have you made any changes to your local Galaxy instance that may have resulted in proprietary datatypes / sniffers not being loaded correctly? What version of the Galaxy distribution are you running? You should be at 6672:e38a9eb21336 from the central repository for the latest tool shed code. However, proprietary datatype support has not been touched in some time.