First, thank you all for Pjiang's useful information and 863211's web link.
I still believe my I-485 is not invalidated.
My situation: filed I-485 with LD in August 07 (EB-2), used AP to come back in July 2008, then to US in December 08. LD works with the same school so she did not use EAD. (We both used AP but not EAD, are we parolee or H1/H4?) H-1 was extended for another 3 years in 2008.
My AP expired in May 2009 and I did not apply for new one. Seemed to me it's easy to reinstate H4 by applying visa at BJ Embassy (according to Pjiang's blog). My understanding is that, AP is a privilege for convenience of travel, but not a must (don't know whether this is correct.) Also, as H-4 with I-485 pending, no rule will prevent me from coming back and forth.
This time my employer (US company in China) applied B1 for me for business trip, when I filed DS-156, I made clear that I have I-485 pending. No problem for visa application.
I believe that's the situation of exception of I-485 rejection: travelling after AP expires. I am in the dilemma here: I have the right to apply for H-4, but the purpose is not as a dependent. And if I apply for B1, I may risk losing i-485. Although AP may save me, it adds extra burden to me (not unusual that AP application takes months. Plus, our company is close to BJ embassy, getting visa for me is easier, and cheaper.
Further, getting to the step of I-485 pending is very difficult, no one would like to lose it, unless they have to, it's common sense.
For the purpose of visit, AP works as greencard, but there might be slight difference, for AP, visit should be related to the basis status that AP is based on (not so sure about this), but again as I said, AP is a privilege, not a must.
Therefore, my conclusion is, for H4/L2 who used AP and then expired, they are either parolee or H4/L2, using B1 to come back to US will not invalidate I-485 pending adjustment. As long as H1/L1 stand valid there, they may reinstate to H4/L2 (even though you used both AP and EAD before).
I would appreciate Pjiang and 863211 to correct me, if I am wrong. I was really sweating when I first got 863211's reply and thought I made a big mistake by not applying for AP and by letting company help me with B1.
Thank you.