I did a bit of research and it appears that a former president and CEO, Kevin Rollins, is in fact Mormon, but he resigned in January 2007 and Michael Dell is the current president and CEO. Wikipedia does note that Dell gave the maximum individual donation to Bush's 2004 presidential campaign, so it is quite possible that he was opposed to Prop 8, but Dell isn't listed on the list of donors who gave more than $5000. Dell also is listed here (http://uts.cc.utexas.edu/~pefsa/DPBreport.pdf) as giving domestic partner benefits, so I have a hard time believing they actively fought Prop 8.
Interesting side note, both Rollins's grandchildren and Dell's children go to the same school I graduated from, which is religious, but comparatively liberal, especially regarding homosexuality.