Is there a workaround for this?

(In reply to comment #1)
> Is there a workaround for this?

Modifying the channel in some other way after adding the comps file will do it
(ie adding a new package or errata). Otherwise you need to get someone to remove
the cached repomd files from disk

To test:
 * use rhn-manage-channel-internal to create a new channel
 * get the last modified date for that channel from the db:
   select last_modified from rhnchannel where label='your-new-channel';
 * Associate a comps file with the channel (the file does not need to exist):
   rhn-manage-comps --server dev --file=some/fake/file -c your-new-channel
 * Verify get last_modified from the db again, and ensure that it has been 
   updated.

requires rhn-content-mgmt-0.5.0 from brew

AO 3/3/08- verified in dev--- it updated the last modified date on rhnchannel

VERIFIED in QA as of 18-MAR