There's nothing built in to do this. 

Personally I would create a global 'speedFactor' variable and divide all time calculations by that value before I used them. Means you have to remember to do it though.

The problem of missing some calls to Update on a fixed timestep is that you will only be able to evenly implement factors of that speed e.g. for 60fps you will only be able to do 1/2, 1/4, 1/3, 1/5, 1/6, 1/10, 1/12, 1/15, 1/20 speeds. If you go this route remember not to use the elapsed time or gametime that is passed in, but assume that you are still on your regular lock step time.

I recommend you use a factor like ZMan suggested. Another problem with skipping calls to Update is that you will make your slow-motion animations look choppy.

Imagine a fast-moving object that can travel across the screen in 10 frames. Now say you want to slow that down so it takes 30 frames. If you skip the Updates, then you will end up drawing it in only 10 different positions, but you will see it in each position for 3 times as long. If instead you apply a factor (1/3 for this example) to your elapsed game time when you do your calculations in Update, then your object will move smoothly across the screen, but at 1/3 the speed.

Hey,

Try this:

protected override void Update(GameTime gameTime)

      float dt = slowMotionScale * (float)gameTime.ElapsedGameTime.TotalSeconds;

      //more stuff...

}

Get your time step from dt instead of gameTime.  You lose a bit of functionality, but you get genuine slo-mo =P

Righteous Tool:

Another problem with skipping calls to Update is that you will make your slow-motion animations look choppy.

For my game I simply have a gameSpeed float variable that I modify. It is always between 0.0f and 1.0f. 1.0 is 100%. I pass the variable into anything in the game that is affected by speed effects.

Some pseudocode would be:

Movement.Y += 3.0 * game elapsed time * gameSpeed;

The ZMan:

Your idea is exactly what I suggested... so no

ooooops..... didn't see that you mentioned it.....

Bit of a late reply, but I was just messing about with this and the following works if you use the gameTime in your calculations:

protected override void Update(GameTime gameTime)
{
    // slowmo 33%
    gameTime = new GameTime(gameTime.TotalRealTime, gameTime.ElapsedRealTime, new TimeSpan((long)(gameTime.TotalGameTime.Ticks * 0.33f)), new TimeSpan((long)(gameTime.ElapsedGameTime.Ticks * 0.33f)));
    base.Update(gameTime);
}