As of April 1, 2015, permits are required for climbs above 4,800ft. This is the standard schedule for St Helens, but in years past the window from April 1 - May 15 allowed for an unlimited number of permits, and now there is a 500-climber limit. That's a lot of climbers, so I doubt it will be an issue, but it is a change this year. May 15 starts the 'high' permit season, when only 100 are available per day. By now, most weekend dates are hard to come by.
Another change for this year: it seems that all permits are online and you need to print them yourself. No more picking them up at the Lone Fir Resort. That's a lot more convenient than having to plan your trip around pickup hours.
From the Forest Service news release:
NEW THIS SEASON:Climbers will need to print their online permits at home and sign in at the trail head climbing registers. Recyclable plastic permit holders will be available at the climbing registers to display climbing permits.Climbing permit must be carried at all times and displayed in a visible manner by each person while climbing Mount St. Helens. PLEASE REMEMBER TO PRINT YOUR PERMIT AT HOME AND BRING IT WITH YOU TO THE CLIMBING REGISTER. There are no facilities available to print your permit at the trailhead.