Is there any video format that is supported by all 4 major browser, IE, Chrome, Firefox, and Opera?
The current selection is Ogg, H.264, and something called VP8 I believe?
safari, chrome and ie will be supporting h.264, while firefox is going to continue supporting ogg. in the near future, the "webm" format from google will likely see support in both chrome and firefox, presuming it remains unencumbered by patents.
firefox's position is being driven by its adherence to free software ideals, so it isn't a matter of waiting for a future version of firefox to join the h.264 supporters. that said, h.264 support for firefox is not prohibited per se, it will simply be handled via plugins which are not installed by default.
so in conclusion, in the near future it looks like encoding to both webm and h.264 is probably the best way to address both camps. there is no single format supported everywhere.
The short answer is no.
WebM will be supported by IE, Firefox, Chrome and Opera.
For more detailed information and a table with supported formats by browser take a look to the "Video on the Web" chapter of the online book "Dive into HTML5".