You are likely to win if you are prepared. Several members of the sfskaters mailing list have successfully beaten skating tickets in court. Here's the experience of a Santa Clara skater:

In October, while returning from a luncheon in Cupertino, CA, I was
stopped by a police officer and ticketed for skating in a bike lane.  

Specifically, the infraction is VC 21966

      Pedestrian in Bicycle Lane 21966.   
      No pedestrian shall proceed along a
      bicycle path or lane where there is
      an adjacent adequate pedestrian facility. 

      Added Ch. 751, Stats. 1976. 
      Effective January 1, 1977.

After having a few days to cool down, I determined that the ticket was
worth fighting and the key was probably the "adequate pedestrian
facility" clause.  If I could demonstrate that use of the sidewalk by
skaters was unsafe, then I would be clear to use the street.

I took my camera out to the scene and photographed narrow sidewalks,
large drops, and blind turns.

I researched state and local laws.  I found that the classic "toy vehicle"
provision doesn't actually exist in California.  Rather the vehicle code
simply defines any human powered conveyance that is not a bicycle as a
pedestrian.  The only place that the word "skate" appears in the entire
code is in the helmet law.

More importantly though, I found that the municipalities that border
the road where I was ticketed both have ordinances forbidding bicycles
from the sidewalk.  Clearly these cities did not see their sidewalks as
adequate for human powered vehicles moving at bicycle speeds.

I was ready.  The court date was set for January 28th.

I came to court in the morning wearing a suit and carrying a brief case
full of supporting materials.  My skates, helmet, and pads were down
stairs waiting to be retrieved by the bailiff as needed.

Several cases came up before mine.  All were the usual traffic issues:
Red light running, speeding.  It was always a choice between traffic
school, fine, or trial.  That meant that was just an arraignment.  I
thought I had a trial.  Crap!  That means I am going to have to come
back!

The judge called me forward.  He looked at the documents with a puzzled
look on his face.  

Judge: "The charge is for a pedestrian in a bike lane.  Were
you walking in a bike lane?"  
My answer: "I was skating in a bike lane."
The judge's expression did not improve.
Judge: "We don't want you on the *sidewalk*.  Case dismissed in the
interest of justice"

I thanked the judge and returned downstairs.  I chatted briefly with the
officers holding my gear.  One officer asked me which department had
issued the ticket.  He looked at the docket in my hand and laughed. 
Santa Clara County Sheriff: his department.

This was easy.  Now Murphy's Law would say that if I hadn't been prepared
then I would have had to fight it for real.

Still, I think my experience shows that skating tickets can and should
be fought.  Don't just roll over.  Just because a particular officer is
giving you a hard time doesn't mean his fellow officers or the judge
will agree with him.  If you fight the ticket and win, the officers may
think twice about ticketing other skaters.  But if you just roll over,
no lesson is learned, you have to pay the fine, and the ticketing is
likely to continue.