Doctor Who Magazine has confirmed it to the Doctor Who News Page.
They don't report this kind of rumor.
Incredibly sad. I wonder if illness was the reason part of Season 5 was filmed at the end of Season 4 last year?
The final episode of Season 4, and the last episode to air in her lifetime, was titled "Goodbye, Sarah Jane Smith", too.
I will be very surprised if they don't do a dedication on Doctor Who, though it's probably too late for them to add anything to this weekend's episode, but you never know.