Is a will invalid if one of the witnesses is an “interested” party? In Florida the answer is clearly NO:

732.504 Who may witness.–

(1) Any person competent to be a witness may act as a witness to a will.

(2) A will or codicil, or any part of either, is not invalid because the will or codicil is signed by an interested witness.

Florida’s is not the traditional approach, which may be why most people instinctively shy away from witnessing wills that benefit them.  For example, Joel Schoenmeyer recently wrote here on his Death and Taxes Blog about a famous pre-WWII case out of California where an “unorthodox” will was invalidated because the attesting witnesses also benefited from the will:

MISS LILLIAN PELKEY’S PETTICOAT
In Los Angeles, before the Second World War, George W. Hazeltine, 86, lay ill in hospital. He wanted to make a new will and leave $10,000 to his nurses, Lillian Pelkey and Madeline Higgins. There being no paper to hand, Miss Pelkey pulled up her dress, placed a board under her petticoat, and the will was pencilled on her undergarment. The petticoat was eventually admitted to probate but the nurses were prevented from benefiting from the will because they were attesting witnesses of it.

By the way, as noted by Joel, Illinois continues to follow the traditional rule disqualifying attesting witnesses from benefiting under wills they witnessed.  

The commentary to Uniform Probate Code section 2-505 (which was adopted verbatim by Florida as F.S. 732.504) explains why the old rule against witness-beneficiaries  was abandoned by the UPC drafters:

The position adopted simplifies the law relating to interested witnesses. Interest no longer disqualifies a person as a witness, nor does it invalidate or forfeit a gift under the will. Of course, the purpose of this change is not to foster use of interested witnesses, and attorneys will continue to use disinterested witnesses in execution of wills. But the rare and innocent use of a member of the testator’s family on a home?drawn will is not penalized.

This approach does not increase appreciably the opportunity for fraud or undue influence. A substantial devise by will to a person who is one of the witnesses to the execution of the will is itself a suspicious circumstance, and the device might be challenged on grounds of undue influence. The requirement of disinterested witnesses has not succeeded in preventing fraud and undue influence; and in most cases of undue influence, the influencer is careful not to sign as a witness, but to procure disinterested witnesses.