Many posts on this subject. The consensus (as you might expect from an online forum) is to go with the online dealers. Savings vs. a local B&M store are usually substantial, as you have noted - especially when you take local taxes into account.
For both B&M and online dealers, any warranty service will be provided by Panasonic, except in rare instances where the plasma is DOA and the B&M store might provide an instant replacement. (Most online stores will also replace a DOA unit, but you may have to pay some shipping costs and it may take a few days.)
Most online sales are for Panasonic's "commercial" plasmas, which have a "carry-in" warranty rather than on-site service. You have to deliver or ship the unit to one of the half-dozen or so Panasonic Service Centers, which are scattered throughout the country.
BTW, if you are purchasing with a premium credit card or an Amex card, the CC company will often provide a one year warranty extension, free of charge. Check your Cardmembers' Agreement.