If a 'green card holder' has never worked in the USA and has been out of the country for long periods (greater than 250 days per year), does such a person have to file a US Tax return? The person already pays tax in another home country.

Chances are that if you re-enter the country they will pull your green card.

That being said the IRS interpretation of the law is this:
Congress recognized that an individual who would be treated as a resident alien under section 7701(b)(1)(A) might be treated as a nonresident alien under the so-called "tiebreaker" rules of the income tax treaties to which the United States is a party. The legislative history makes it clear that Congress did not intend for section 7701(b) to override any treaty obligations of the United States:

The conferees do not intend that the conference agreement override treaty obligations of the United States. For example, an alien who is a resident of the United States under the new statutory definition but who is a resident of a treaty partner of the United States (and not a resident of the United States) under a United States income tax treaty will be eligible for the benefits that the treaty extends to residents of the treaty partner.

Figure out what it is you want to do with yourself.

If a 'green card holder' has never worked in the USA and has been out of the country for long periods of time (greater than 250 days per year), does such a person have to file a US Tax return? The person already pays tax in another home country.

If your total worldwide income exceeds the filing requirement, you must file your U.S. tax return. On your return, you must report your world wide income. If you paid taxes in the foreign country, you will claim credit for taxes paid in the foreign country by filing Form 1116 or can file form 2555 Foreign Earned Income Exclusion.. Read: Your U.S. Tax Return: U.S. Citizen or Resident with Foreign Income (http://taxipay.blogspot.com/2008/03/us-citizen-or-resident-with-foreign.html)

In short, you have filing requirement but it most cases you will not pay any U.S. tax or pay very little U.S. tax.

This from Pub. 519:

Effect of Tax Treaties
The rules given here to determine if you are a U.S. resident do not override tax treaty definitions of residency. If you are a dual-resident taxpayer, you can still claim the benefits under an income tax treaty. A dual-resident taxpayer is one who is a resident of both the United States and another country under each country's tax laws. The income tax treaty between the two countries must contain a provision that provides for resolution of conflicting claims of residence (tie-breaker rule). If you are treated as a resident of a foreign country under a tax treaty, you are treated as a nonresident alien in figuring your U.S. income tax. For purposes other than figuring your tax, you will be treated as a U.S. resident. For example, the rules discussed here do not affect your residency times as discussed later under Dual-Status Aliens.

Information to be reported. If you are a dual-resident taxpayer and you claim treaty benefits, you must file a return by the due date (including extensions) using Form 1040NR or Form 1040NR-EZ, and compute your tax as a nonresident alien. You must also attach a fully completed Form 8833 if you determine your residency under a tax treaty and receive payments or income items totaling more than $100,000. See Reporting Treaty Benefits Claimed in chapter 9 for more information on reporting treaty benefits.

If you are resident in a country who is a US treaty partner you file a 1040NR and do not report income sourced without the US.