Travel Insurance International Reviews

Is Travel Insurance Mandatory for Travel to USA?
My parents are travelling to USA in feb, 2008. I just want to know whether travel insurance is mandatory for international travellers coming to USA on family visits?
If yes, may i have some insurance plans from reputable companies (at lower cost) which have good customer reviews?
I heard that "Travelers Liberty" plan is quite cheap and good for short trip, but don't know about the reputation and claim settlement efficiency of the company who is offering this plan.
Not exactally mandatory however, I believe that there is a requirement that the visitor have enough resources to be financially responsible.
Best place to find travel insurance is at a travel insurance comparison site like http://www.quotewright.com
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