You can now see percent increases or decreases for Clicks from search, Appeared in search, Pages crawled, Crawl errors and Pages indexed. Here is a screen shot:
The way it works is that it will take the date range you specified, plot that data in the current period data and then look at the same time frame in the period before. For example, if you look at the past 30 days of data, it will compare that data to the 30 day period before that.
Couple technical notes:
(1) With the pages indexed, Bing will show you the percentage change between the last day of the selected period and the last day of the prior period.
(2) Bing's Webmaster Tools data goes as far back as 6 months, so you can compare 30 days, one day, or even 3 months of data.
Finally, Google introduced this feature around October 5, 2010.
Forum discussion at Google+.