Both Seattle Likes Bikes and the Seattle Bike Blog have great posts highlighting the Dexter changes that started on Monday. Long-term, there's a lot of benefits to cyclists, from the SDOT Project Page:
This project will improve the pavement condition by repaving the street. As part of the repaving project SDOT will:
In return, there's some short-term pain. Construction will run throughout the spring and summer, concluding in September. The Seattle Likes Bikes post has details on the expected conditions and alternate routes, based on the presentation at this month's Seattle Bicycle Advisory Board. The Seattle Bike Blog post has the SDOT flyers, quite useful.