Is it possible to run Microsoft Excel under Ubuntu?

Your best option is to install a copy of Windows (preferably XP as it is much more smaller footprint) in a virtual machine (such as VirtualBox ) and install your copy of Excel inside that, from there you can run the virtual in seamless mode where it crops out the entire background and leaves just the running application on the screen. Quite beautiful actually.

I doubt that you will have much success using a product such as Wine as because the latest office suite relies on the .NET framework that is spotty supported only up to V2 (as opposed to the latest is V4.x)

There is then the commercial backers of Wine called CodeWeavers which support such products on a more professional level... but that is for a fee, and also I dont think even they are up to the latest MS office suite support yet.

EDIT 6-22-2011:

There are a few more options that I have thought of just recently:

With Microsofts latest office release I do believe they now have web apps that function fairly similar to the actual application albeit with limited functionality, but this may give you the flexibility and comfort you seek with the compromise of power.

If you do have windows installed on a home computer, such as a HTPC, you could install you version of Excel on that computer and then RDP/VNC into that computer from Linux and use Excel from there. This would work decently since excel is all text and little graphics.

The last option would be to explore other web applications that offer excel-like products which could offer you comfort better than libre office, and more like excel. I know that Google docs has a web application for excel content, along with completely cloud OSs such as eyeOS

You can run a previous version of Excel using wine.

You can use Play On Linux to easily install Microsoft Office on Ubuntu. Everything is well explained here:

http://www.makeuseof.com/tag/easily-install-microsoft-office-2007-linux/