Some players have been experiencing a lot of lag while playing Diablo 3 on PC since as early as the weekend.
A large number of reports have popped up on the game's technical support forum over the weekend and throughout today. The reports are of major lag, even during solo play (which requires an online connection still).
Players are reporting pings in excess of 2000ms, rendering the game unplayable.
Blizzard has responded in a forum thread, saying that it's currently collecting data to try and determine the root cause. A support forum agent posted a few diagnostic steps for anyone suffering from said issue, which basically try to simulate the data travel path from Blizzard to the user's PC and vice versa.
Head through to the link, and follow those steps if you're having any connection issues.