Yes, taking the pitched speech samples is the way to go. However...

I would go a bit further and, besides speeding up the sample, I'd also tweak the formants.

Personally, I think that manipulating the formant could help you adjust the voice, so it would match the character appearance. You know, like a fat character, or tall and thing, or small and cute. By tweaking the formant, you wouldn't have to rely on additional pitching, which could be more time- and resource-consuming.

There's another technique for "gibberish" dialogue called Grammelot, originally used in theater, which I think is a bit more sophisticated than a procedural solution; it also produces incomprehensible dialogue.

You can hear it in games like Rayman 2 (please head to 1:40):

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The technique consists of having the actor speak by joining random phonemes that recall to a certain language. It feels like talking - especially when the actor conveys emotion and intonation as he performs - but the message won't mean anything.
In case of Rayman 2 it sounds like french grammelot (probably related to the fact that the game was developed in France...).