How to gradually slow down/speed up audio tracks in Logic Pro 8?

I assume you are working with audio.
I also assume that this audio was not originally recorded in Logic.
Therefore the first step must be that you bounce/export all individual audiotracks from Logic, and reimport them to the project, replacing the original audio files. On the reimported tracks, check the "follow tempo" checkbox (in the Inspector) on every audio track. Tracks that already are able to follow tempo (=have a Follow Tempo checkbox) do not need to be ex- and reimported.
Now start your project with a very low tempo (use the Tempo Track in the Global Tracks) and create a tempo curve that leads up to the eventual tempo you want it at.
Done.
In this example it is the other way round, it starts with an absurdly high tempo and falls back - the principle is the same though:
!http://farm5.static.flickr.com/4021/4340356576 214907c279o.png!

regards, Erik.

'Follow tempo' only shows up for audio files that were recorded within logic.

In any case, I'm not sure this is the best way to go about creating the effect you want. What I'd probably do is record or bounce the starting part of the music (the bit that I want to apply the pitch wobble effect to), and then import it into the sampler (EXS24). Then I'd trigger it and use some pitch bend, tweaking it until I got just the right kind of tape wobble effect that I was after.

Much easier to do all of this in Logic 9.1 btw.. You can instantly and automatically create a sampler track from an audio region. And also in Logic 9.1, there are now 'speed fades' as well as the usual fade in/out/crossfade. So you can just quite easily create all kinds of pitch warp effects.

You just need to create a sampler instrument with the EXS24 and put your audio into it, so you can play it with a keyboard just like in any sampler. Then you just trigger the sample, and record it as midi on an audio instrument track, and use your pitch bend wheel to wobble the pitch as desired.

This really is a lot easier in 9.1, which is why I suggested it. You can automatically create an EXS24 instrument using an audio region, and have it magically appear right in front of you in your arrangement. It does it all for you, you don't need to learn how to use the EXS24 first. So unless you have a reason for not wanting to upgrade, it's probably worth the 200 bucks. You can always keep using Logic 8 afterwards, if for some reason 9.1 doesn't work well for you.