Can I use a different brand of underlayment under my Pergo laminate floor and still be covered under the warranty?
It appears you can use any brand of underlayment as long as it meets Pergo's specifications. Their warranty is concise and straightforward and the only reference to an underlayment is made in the "Conditions" section, and it reads as follows:


  Foam underlayment should meet or exceed 2.0 lb polyethylene foam (1.9-2 lb density), CFC Free, 1.8mm to 2.1mm thickness. 


By the way, according to Pergo's installation instructions, an underlayment is required only when it's not attached to the flooring.