Actually...it happens all of the time on the late night shows and occasionally on the network morning news shows. When Jennifer Love Hewitt appears on Jay Leno's show, he mention's Jennifer's show "Ghost Whisperer" airs on CBS.
And the network news anchors have appeared on rival networks late night shows before. CNN, MSNBC, and Fox Noise anchors appear as well (even on the morning shows produced by the network news divisons). Likewise, network anchors frequently are guests on CNN and MSNBC.
"The View" plugs rival shows as well.
Oh...and ads for A&E and History Channel air on the Turner networks and The Weather Channel (not just during local ad-insertion periods). Once, the History Channel bought an ad package on The Weather Channel, and they put the History Channel logo on the Local Forecasts (if your cable provider has the Intellistar).
Then there is the TV Guide Network, which is (albeit lesser and lesser) a clearinghouse for promoting other networks.
If your local TV station is co-owned with a radio station cluster (like Cox and WSB-AM/FM/TV in Atlanta), you hear ads for rival TV stations. For example, WSB-FM/B98.5 has more ads for WXIA and sister WATL, with studios next door to the WSB complex, then for WSB-TV, whose studios are two floors up. Same for TV/ WSB-TV has more ads for WUBL/The Bull than for any of the five co-owned stations with studios two floors down. (oddly, WUBL's previous format until last year competed with WSB-FM, but that's another story).
And the newspaper (again, Cox in Atlanta with the AJC, under cross-ownership rule waiver due to grandfathering) will run ads for other TV and radio stations.
And CBS owns billboards. I find WSB-TV (ABC) and WXIA/WATL (NBC/My Network TV) ads on them all of the time.
The only prohibition of cross-promotion is in local TV (and that is because TV station owners write the prohibition into talent contracts for reasons I don't know). Even a radio DJ working at one station can appear on-air at one owned by someone else (in most cases, I know that the WSB-AM talkers have appeared on Cumulus stations, of course the same noncompete rules generally apply after a contract experation).
Promotion of rival networks happens every single day. Most people just don't pay that much attention.