From what I've been reading about US patents, once a patent has expired, it enters the public domain and can be freely used.
The patent I was looking at was filed on 1/1999 and published in 10/2000. So, obviously the 20-year patent term has not yet be reached. However, the status appears to be expired due to non-payment of the maintenance fee.
On google patents, the status is:
12/2008 Expired due to failure to pay maintenance fee
On uspto.gov, the patent's status is shown as:
Patent Expired Due to NonPayment of Maintenance Fees Under 37 CFR 1.362
Has the patent entered the public domain and can it be used freely?
Additionally (and assuming that it is in the public domain), can someone come along at some point and start paying the maintenance fee again and pull it out of the public domain until the 20-year term is reached? It is very unlikely that this patent will be revived by the company that originally filed it since they shut down production of products using the patent in 2000 and moved onto other things.