akt77,
My error seemed to be a memory issue. I'd seen this before with CS4 occasionally, when all of a sudden it went into Outline mode and wouldn't let me back out to Preview because I'd been working too long without saving. (I think at the time I upgraded from 2GB to 4GB RAM, and things got better. I've been running CS6 now for a couple months, and this is the first time it's popped up again.)
I was not using Transparency. I closed every single other program on my PC (didn't resolve issue). I reduced design size by 50%, then another 50% (didn't resolve issue). I flattened everything (didn't resolve issue). Then I selected the most complex part of my design that had been live traced, and went to Object > Path > Simplify > Curve Precision > 99%. That reduced the points from about 300k to 42k and resolved the issue! Was able to Preview then Save as normal.
Good luck,
RBwebbie
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Windows XP Home
Intel Core2 Duo 3.00GHz, 3.5GB RAM
Saving to D: data-only drive with 340GB free space