It has been a while since I produced the last delegate estimates for the upcoming Democratic primaries. Part of the reason for that is because the upcoming Democratic primaries are still several weeks off. Another reason is that there really hasn't been enough polling in several states such as Indiana, West Virginia, and Kentucky. However, a slew of new polls have come out in the past week or so, providing pretty good coverage of the upcoming primary states (except for Oregon, where there has not been a poll since January).
In Pennsylvania and North Carolina, we have several polls, so I use the Pollster.com averages. In Indiana, West Virginia, and Kentucky, I use the most recent poll in each state. Based on these polls, Hillary Clinton would pick up about 230 delegates through the May 20th primaries compared to 194 for Barack Obama (with 52 delegates in Oregon listed as "unclear" because there is no recent survey data available).
This means that by May 20th, Clinton will have been able to cut about 36 delegates off of Obama's pledged delegate lead (which currently stands at 162). That means he would still have a lead of more than 100 pledged delegates by May 20th (with only three states voting after that date).
On Monday, I used the Obama campaign's delegate predictions to estimate that he would clinch a majority of pledged delegates on May 20th. The poll-based delegate predictions lead to a similar conclusion. If you assume the delegate allocations predicted above from the state-by-state surveys, and then split Oregon's delegates evenly (since we have no polling data on the state), then on May 20th, Obama would have 1,637 pledged delegates, 10 more than he needs to clinch a majority of pledged delegates (I am assuming Obama will win 2 of Guam's 4 delegates on May 4th).