This is still a developing area. One that has had some heated debate on the boards.
As time has gone on, I've started to see things a little differently.
Apparently at one point in time there was a rule that one had to stay with the sponsoring company for at least one year. However, that was struck down years ago and was replaced with intention to work with sponsoring employer upon greencard approval.
Before Yates came out with his in august 2003, some were speculating that uscis was going to come out and say that after 485 has been outstanding for more then 180 days then we are free and clear of getting the greencard, no matter what the current job situation is when uscis gets aroound to adjudicating the 485. However, uscis came out a little bit more strict and expect you to be in same/similar job at the time 485 gets approved.
USCIS was supposed to come out with regulations on ac21 and they haven't in five years. I think they haven't come out with it because there are conflicts within the rules and there are a lot of loopholes that people could exploit. Therefore, they would have to overhaul a lot of code of federal regulations at the same time to close the loopholes.
Just to give you an example:
ac21 states that you can change or change intent to which employer you want to work for as long as it is same/similar job after 485 o/s for more then 180 days. It doesn't say anything about you actually being employed in that same/similar job when 485 gets approved. You could almost interpret this that after 180 days you are free and clear.
The 140 employer is the sponsoring employer and not the ac21 employer. If you are supposed to work with the sponsoring employer of the 140 but can change after 180 days, the new employer isn't the one who sponsored you, so therefore, they haven't made any certifications to department of labor (labor certification) or on the 140 to employ you upon greencard approval. If the new employer isn't considered the sponsor then you could leave at any time.
USCIS regulations state that there has to be an approved and valid 140 immigrant petition for a person to adjust status. However, employers have been revoking them and people are still adjusting status with ac21. There are other regulatins that state if employer asks for revocation then uscis has to revoke it. This is another conflict that uscis will have to resolve.
Now back to your questions. I'm not aware of anyone who has gotten in trouble for leaving the job too early. However, keep in mind ac21 is only five years old and long delayed 485 processing has only existed since 2002. Therefore, a lot of the people who may have changed jobs too early would not have gotten onto the radar of uscis yet.
The main guidance I have seen is, how does uscis look at "intent". It appears that if you stick with the 140 employer or ac21 employer for three months after greencard approval then you shouldn't have any problems.
According to AILA liaison meetings in March 2005, uscis is working on ac21 regulations and it should shed some light on all these issues within this year.