Here's the law, according to the Family Medical Leave Act (posted through the Department of Labor's website):

Covered employers must grant an eligible employee up to a total of 12 workweeks of unpaid leave during any 12-month period for one or more of the following reasons:

for the birth and care of the newborn child of the employee;
for placement with the employee of a son or daughter for adoption or foster care;
to care for an immediate family member (spouse, child, or parent) with a serious health condition; or
to take medical leave when the employee is unable to work because of a serious health condition.

Employers who are covered include:

An employer covered by FMLA is any person engaged in commerce or in any industry or activity affecting commerce, who employs 50 or more employees for each working day during each of 20 or more calendar workweeks in the current or preceding calendar year. Employers covered by FMLA also include any person acting, directly or indirectly, in the interest of a covered employer to any of the employees of the employer, any successor in interest of a covered employer, and any public agency. Public agencies are covered employers without regard to the number of employees employed. Public as well as private elementary and secondary schools are also covered employers without regard to the number of employees employed.

So, as long as your employer fits into those parameters, you are entitled to leave of up to 12 weeks. As far as whether or not that leave is paid, however, is a different story.

In this great article in the NYTIMES, "Breaking the Biology Barrier," about how some workplaces are starting to treat adoptive parents like biological parents:

With more than 100,000 Americans adopting each year, adoption benefits are becoming a hot new perk in the panoply of workplace benefits. Whether paid time off, reimbursement for costs or both, the benefits help parents defray hefty adoption fees and afford bonding time with new children. Just as important, recipients say, the assistance sends the message that adoptive families are as valued and worthy of support as biological families are.

Not all workplaces are supportive, of course, and many people work for companies well below the 50-employee cut-off. In addition, many people are self-employed, which makes it difficult to take longer than a few weeks off to be with the newly adopted child.

How did your employer treat your adoption? Did you have to fight for your rights to parent?