Numerous reports on Twitter indicate that a rocket from Hamas has hit Tel Aviv. Here's one from Israel's ambassador the U.S., Michael Oren:

ALERT: Tel Aviv area was just hit by a rocket attack from #Gaza. #IsraelUnderFire

— Amb. Ron Dermer (@AmbDermer)
November 15, 2012

And here are more tweets, confirming Oren's report:

Air raid sirens sound in Tel Aviv, explosions heard - witnesses

— Reuters Top News (@Reuters)
November 15, 2012

Reports of explosion in Tel Aviv, #Israel. Air raid sirens heard.Residents ordered into bomb shelters#stopthereckets #Hamas

— Josh Block (@JoshBlockDC)
November 15, 2012

One rocket hit Tel Aviv, no casualties according to Israeli Army radio. Watch http://t.co/NzQPj2jZ for updates

— Reuters Top News (@Reuters)
November 15, 2012

UPDATE: The Israeli military reports that "no rockets struck in the area."

A few minutes ago, sirens in Tel Aviv sent residents running for shelter. We can confirm that no rockets struck in the area.

— IDF (@IDFSpokesperson)
November 15, 2012