I'd guessed that the Jackson Memorial coverage would top 30 million viewers, and it just barely did.
A memorial service held in honor of Michael Jackson drew more than 31 million television viewers in the U.S. The event was carried live from approximately 1pm ET to 4pm ET on 19 networks: ABC, NBC, CBS, FOX, Telefutura, Telemundo, Univision, CNN, Fox News Channel, MSNBC, Headline News, BET, E!, MTV, VH1, VH1 Classic, TV Guide Network, and TV ONE.
via -nielsen wire.
Update: Here is some additional data from nielsen wire.
Update: And via MSNBC link from commenter RG, here's some reference cumulative audience comparisons to other recent big televised events.
Obama Inaugration, January, 2009 – 38 million (multiple networks)
2008 (August) Summer Olympics (Opening Ceremony) – 34.2 million (one network)
2009 (May) finale - 28.9 million (one network)