In 2002, Toyota offered an extended warranty to some owners who were affected by the sludging problem, but many customers claim to have been accused of improper maintenance and were forced to pay for their own repairs.
Sludging is the result of several factors, some of which included longer drain intervals, tighter engine tolerances, and the typically higher operating temperatures of new engines. Several other manufacturers have also faced allegations of oil sludging problems and have responded in a variety of ways.
As a consumer, the best defensive measure is, of course, a regular oil change (following the "severe service" schedule if required) with a name-brand oil that meets the manufacturer's recommendations for viscosity and API service rating. Some manufacturers recommend different viscosities for operation in extremely hot or cold climates; as always, the owner's manual is an outstanding source of information on matters like these.
[Source: Consumer Affairs; a hat tip to AB readers Drew and Ryan]