These rules impose reporting requirements on a disposition of ESPP shares that occurs after you have held the shares long enough to avoid a disqualifying disposition. Unlike the rules for incentive stock options, these rules may require some or all of your profit to be reported as compensation income even after you've satisfied the holding period requirement. The amount of compensation income is calculated differently than for a disqualifying disposition, using the lesser of two numbers. Strangely, it is possible (although unusual) for the amount of compensation income to be larger for a qualifying disposition than it is for a disqualifying disposition.
The law on this issue is poorly written and causes plenty of confusion among the companies that maintain these plans, the individual participants and their tax preparers. Even the IRS has sometimes appeared to be confused about this rule, stating it incorrectly in Publication 525.
for more info on ESPP, check out:
http://blog.wenxuecity.com/blogview.php?date=201104&postID=7381