According to the FAQ they are - see the “System Requirements, Integration, and Compatibility” section, here:

http://www.microsoft.com/office/preview/programs/excel/faq.mspx

However, when I was challenged recently by a customer as to exactly how backwards compatible they were regarding the new and improved Business Intelligence capabilities, I thought I’d better do some further investigation and this is what I found ...

Take a look at the following spreadsheet in Office Excel 2007, nice eh?  A lot of meaningful numbers but impossible to tell at a glance as to where the big number is, where the small number is...

With the tiniest bit of effort, Office Excel 2007 allows you to mark up the spreadsheet rather nicely in a visual format - check out the Conditional Formatting option on the Ribbon...

After we've nicely marked it up (I will post a link to a video on how to do this soon :), we're left with a couple of options when it comes to saving the file...

We could save it in its original format (The file opened was .xls, Office Excel 2003 format). This option lets us know that we're going to have a 'significant loss of functionality'...

Or, we could convert it to Office Excel 2007 format...

Office Excel 2007 is kind enough to let us know that if we do this, we can use all of the new features and reduce the size of the file...

So, I did both :) which left me with a full fidelity .xlsx file and an original .xls file...

Opening the .xls file in Excel gave me the exact same result (so nice and consistent) as doing the following steps.  First of all, Office Excel 2003 told me that the file had been created by a newer version and asked me if I wanted to download a converter...

Then, it took me to the Office Preview website where the file converters live...

It guided me through installing the first update required, a file format awareness update...

Then, it guided me through installation of the Office Compatibility Pack...

Following installation, when I double clicked the .xlsx file, I got a brief conversion in progress dialog followed by the following message, letting me know that the file had been opened read only to ensure that nothing in the original file would be lost due to compatibility issues...

 

Finally, the file looked like this when it was opened (not that this is the same for the .xlsx with the file converters installed and the version saved as .xls)...

So, to conclude, it appears that new features that exist only in Office Excel 2007 only work in Office Excel 2007. However, the files, to the best of their capabilities are backwards compatible. Where’s the justification for this though? Well, if I brought a brand new car with air conditioning and a CD player fitted as standard, I can imagine the response that would be given if my mate, who, for arguments sake, had purchased an older model, would get if he took it back to the dealer and asked why he hadn’t got it!!! So, I guess this is to be expected...

Be sure to check out the Office Preview website for all the latest information on the 2007 Microsoft Office System:

http://www.microsoft.com/office/preview/default.mspx

For a comprehensive FAQ on the Office file formats take a look here:
http://www.microsoft.com/office/preview/itpro/filefaq.mspx