Most, if not all, large companies are running either XP or Windows 7. Most will be running either IE7 or IE8. Many will not be running IE9 and hardly any at all will be running IE10.
There will be a lot of staff using FireFox and Chrome, but it would hard to believe that all of those version will be supported by all of the big corporations.
Things are now likely to change. IE10 has just been released for Windows 7. The IE10 browser is SIGNIFICANTLY better at rending the new HTML5 standard. IE7 was terrible, IE8 was no better. IE9 wasn’t much good and IE10 is a step change. It brings with it a lot of its own problems – such as not being able to render Outlook Web Access 2003, but that is a small issue compared to the benefits it will bring…
Some large companies use the Chrome plugin for IE9 and below to better render the HTML5 code. With IE10 now available, might we see a step change in corps for them to move to that new version? Given the hard push for multiplatform software I suspect HTML5 and IE10 are going to be the way forward.