+1 for this request. I just posted a similar (perhaps redundant) post in the forums:
http://forums.omnigroup.com/showthread.php?t=31186
I really like the way Workflowy has implemented tags/filters and hope the OO4 could do something similar.
I may be wrong, but I feel like that company saw what omni was doing and basically tried to ape it to create their own web-based outlining platform. So I don't think Omni should have any qualms about aping them back!