Underlining the importance of the expanded "underwriters" messages on PBS, the public broadcaster is expected to announce today (Monday) that it has subscribed to the Nielsen ratings service, the New York Times reported today. In the past, companies often joined charitable trusts to support programming on PBS as a public service and for the prestige of being associated with esteemed and influential productions. They received barely a mention on the air. Each underwriter's announcement may now last up to 15 seconds, and although they are barred from promoting the underwriter or its products or services, some appear close to crossing the line. Since public broadcasting stations often do not carry programs at the same time or even the same day across the network, PBS will be subscribing only to the weekly Nielsen ratings, not to the Nielsen "overnights."