True to an extent
Immi2001,
It's true to an extent that GC holders cannot stay outside of US for more than 6 month period. This is critical when your physical stay in US is looked at, if and when, a person applies for US Citizenship. In other words - Any stay longer than 6 months could become a cause for INS to believe lack of interest in maintaining US residency. If the individual doesn't plan on applying for US citizenship the stay can be extended upto an year or two with prior approval (Re-entry Permit).
You should be able to find information on the INS website, try searching for re-entry permit.
Hope the info helps you.
ucn.