Here is the scenario:

We have a cold case murder from about 25 years ago, and we have a tip that has led investigators to Florida in search of a suspect. WE do not want to tell the suspect that we are investigating the cold case for fear that he will not give us a DNA sample. Is it permissible to lie to the suspect and tell him that we are investigating a Florida case in which the suspect in the fictional case works for the same company as the cold case suspect in order to get a voluntary DNA sample? Will that stand up if the consent is based on a lie?

Any help would be greatly appreciated.

You would probably be safer collecting something the defendant has abandoned than use trickery.

Still, there is support for your plan:

State v. Athan, 158 P.3d 27 (Wash. 2007) (Athan first argues his DNA (deoxyribonucleic acid) was collected in violation of both the United States and Washington State Constitutions when Seattle Police Department detectives, posing as a fictitious law firm, induced Athan to mail a letter to the firm, from which Athan's DNA sample was extracted.)

Wyche v. State, 906 So.2d 1142 (Fla.App. 1 Dist. 2005) (2-1) (salvia lawfully obtained where: police Department Investigator . . . asked Wyche for a saliva sample, stating that he was suspected of committing a burglary at a Winn-Dixie supermarket. In fact, VanBennekom had manufactured the fictitious Winn-Dixie burglary in order to obtain Wyche's consent to take swabs for a sexual-assault investigation.), review granted 918 So.2d 294 (Fla. 2006)

But see State v. McCord, 833 So.2d 828 (Fla.App. 4 Dist.2002) (Unlike the officers in Frazier and Washington, who did not lie to the defendants regarding the crimes for which they were suspects, the detective in this case fabricated a rape charge to obtain McCord's consent. Even the detective testified that McCord consented to giving a sample only because he wanted to clear his name in a non-existent case. We agree with the trial court that this deception, while McCord was in jail, was so manipulative that his “consent” did not “validate the search.).

While this seems to be a constitutional issue, you might also think about whether there is any Florida law that this would violate. Davidson v. State, 25 S.W.3d 183, 186 n.4 (Tex. Crim. App. 2000) (discussing art. 38.23 and conflicts of law issues).

Here are a couple more cases to consider:

United States v. Flynn, 309 F.3d 736, 739 (10th Cir. 2002) (holding ruse created to cause defendant to abandon item was not illegal); People v. LaGuerre, 29 A.D.3d 820 (N.Y. App. Div. 2006) (holding no due process violation when DNA sample taken from chewing gum defendant discarded in course of police contrived Pepsi taste test).

State v. Christian, 723 N.W.2d 453 (Iowa App. 2006) (not published) (no due process violation where "[Police officer] arranged to sit in on the [job] interview to attempt to obtain a DNA sample from [suspect] by furnishing him with a bottle of water that would be subsequently retained for DNA testing.")

Commonwealth. v. Gaynor, 820 N.E.2d 233, 243 (Mass. 2005) ("In those cases [where consent was ruled invalid], consent was negated or limited by a defendant's reliance on the express representation of a police officer that the particular object to be seized would be used solely for a particular purpose, and therefore a warrant was required for a search and seizure for purposes beyond the self limiting representation of police. See 3 W.R. LaFave, supra at � 8.2(n), at 708. Here, police made no express representation that the test results of the defendant's blood would be used solely for comparison to the tests on blood found in the fourth victim's car, and the judge so found.).

Thanks for the help. Our detectives ended up following him to a Chinese restaurant and got his utensils, etc. after he ate, so we should be able to go from there. Thanks again for your help.

Keep us posted on the results!

Interesting article in the NYT today about this issue. The defense push back is:

Some legal experts advocate curbs on surreptitious sampling. Albert E. Scherr, a professor at Franklin Pierce Law Center in Concord, N.H., who has a grant from the National Institutes of Health to study the practice, suggests that the police be required to meet the "reasonable suspicion"; standard before secretly collecting DNA. "You';re not asking them to let criminals go free,"; he said. "You're just asking them to investigate a little more."

http://www.nytimes.com/2008/04/03/science/03dna.html?ex=1207886400&en=7a7d58f559318e75&ei=5070&emc=eta1

(you may need to be registered at the NYT to open the link)