Should be able to. None of the applications I test here were written with .Net. All 5 that I work with are written in various versions of Java/JSP/PHP, and run on WebLogic and JServ. When testing initially, I ran against Yahoo...though with only 1-2 users. Additionally, they may have the robots.txt file with the exclusion data in it, in which case you'd need to change the test settings. HOWEVER, I'd strongly caution against stress testing Yahoo, or any other site where you do not have express permission...I don't think they'd like that (it'd look a lot like a DOS attack, since it all comes from one IP address).
I prefer 'maliciously mischevious'.
TDAdmin of Evil