Okay, I've been wondering a few things. Can Kitty phase through any forcefield? Is there a kinds she's failed to get through? Being that Magneto's is electromagnetic I would ASSUME she should be able to phase through it with ease, right? 
 
Side question: When she's phasing, can she still be levitated by certain means? Can her mind still be assaulted psychically or in some way physically like say someone controls the bio-electrical signals in there?

"    Well, this is what I THINK I know about her. She can phase through force fields, but not as easily as she can solid barriers. She's had trouble with Magneto's force field, for example...and he was able to knock her out once when she tried to phase through him. In fact, he was afraid that he'd killed her.     I don't know about levitating, but I'd assume that she couldn't me levitated or held unless she let it happen. I know for a fact (it's been talked about) that while phasing, she is resistant to mental attack. For example, Emma Frost couldn't seem to do much to her when she was in phase, but her resistance didn't help much when Cassandra Nova - a telepath at least as powerful as Charles Xavier - attacked her. "

Hm .. I'm not too sure about this. :/
 
But I know she's highly resistant to mental attacks.
Because when she's in phasing mode, she technically has no brain to be affected. Which I agree with, so if telepaths can affect her in her phasing state, it should be an error.

depends on the nature of the force field. 

she only is changing her density. but she is still intact structurally.
this is why Magneto's field works on her. Magnetos field is a manipulation
of the polarity and intensity of the electro-magnetic fields present around him.
basically its like a scrambled code, that several things cant pass through or have
a hard time doing, one the number one things being electrons themselves,
which regardless of being phased, Kitty still has in her composition.
 
personally i always wanted to see her tested against Sue Storms field, which has stopped and
contained a phaser before ( Iconclast, probably the best chance she had to have an arch-nemesis
here, unfortunately the writers seem to have forgotten him).

Could Kitty resist/be immuned to a telepathic attack on the level of say, Xavier or Jean Grey while in her phase-form?

To comment on the Kitty phasing's against Sue's force fields. During Ultimate X-men/Fantastic Four, Kitty is in phased state and Sue puts a field behind Kitty. And instead of going through the field she hits her head on the bubble and knocks out. When Bobby and Wolvie come to Kitty, she is unconscious and stuck in her phased state.....So, if that is correct and wasn't an error on the writters part, than Kitty is effected by Sue's force fields. 

There was an issue of Excalibur against Dr. Doom, where she couldn't phase through Doom's forcefield.  Doom's field might be sufficiently advance to resist phasing powers compared to other fields.