There are many threads across the web with webmasters concerned that their sitemaps file is in "pending" status in Google Webmaster Tools. One such thread is at Google Webmaster Help where a few webmasters are complaining about the issue.
Google issued an announcement about the "pending" Sitemaps status. Google said it is normal;
Most sites that submit Sitemap files are regularly seeing a "pending" status in Webmaster Tools for their Sitemap files. This is normal and to be expected. It shows that we are planning on fetching your Sitemap file again and that we will update the statistics as soon as we have done so. This is not a cause for alarm. Thank you for using Google Webmaster Tools and submitting Sitemaps.
That is one reason why the Sitemap file might be in "pending" status, but there are others. JohnMu explained there is a "Sitemaps processing pipeline" that has a priority queue based on Google's available resources at the time. Things that can slow the crawl of a Sitemaps file include "servers with slow response times" or "sites that do not meet our [Google] Webmaster Guidelines."
Forum discussion at Google Webmaster Help.