As of 05/08/2010, the hole is still there.

On 05/02/2010, I emailed Philly311 and cc’d Mayor Nutter.
philly311@phila.gov
mayor.nutter@phila.gov

Here’s the response from Philly 311:

Wed, May 5, 2010 2:30 pm

Dear Mr. ChuckWagon,

Thank you for submission to Philly 311's website. We have entered a Service Request under ticket # 939972 and have forwarded it to Fairount Park Commission for action.
Thank you again for using Philly 311 and we look forward to assisting you in the future.

Stacy P. Tutis, Supervisor
Philly 311 Contact Center
Room 153 City Hall
Broad & Market Streets
Philadelphia, PA 19103
215-683-0866
215-683-0883 (fax)

You can call the Fairmount Park Commission District 3 office to inform them about the hole in the bridge. Their number is 215-685-2575.

June 4, 2010 - a stop sign now covers the hole. It looks like a temporary fix.

Ticket #1031132. Ticket forwarded to Fairmount Park for follow-up. (mb)

July 4, 2010 - This doesn't look too permanent or professional, but it should "STOP" the problem.

The bridge will be fixed according to Sarah Stuart of the Bicycle Coalition. Parks and Rec will start work on the canal trail in the near future and will address problems with the bridge.

09/18/2011 The STOP sign is still there! I guess "the near future" means a year os so. And the fallen trees (in the background) are still there (see
http://www.seeclickfix.com/issues/123577-fallen-trees-blocking-manayunk-canal-path).
It’s hard to believe but it looks like some work has been done to make it easier to get around and through the trees. Maybe they'll be removed in "the near future" too!