Daily, I'm continuing to receive several PDF based stock messages that are being massively spammed.  Both the ISC and Avert labs are warning that Excel document types commonly used in the business environment are now being used. AVERT suggests that Word and other Office formats might also be used in the future to circumvent corporate attachment blocking rules. 

Stock Spammers now sending Excel documents
http://isc.sans.org/diary.html?storyid=3177
http://www.avertlabs.com/research/blog/index.php/2007/07/24/pdf-spammers-already-moving-on-to-other-filetypes-currently-xls/

QUOTE: PDF spam has continued to increase during the last 3 weeks and has moved from ‘pump and dump’ stocks to other types of spam such as pharmacy spam.  The spammers responsible for the recent .PDF based ‘pump and dump’ stock spam have also started to send pump and dump spam containing Microsoft Excel .XLS documents