When you receive an Excel spreadsheet as an attachment in Gmail, you'll see a new option next to "View as HTML" and "Download": "Open in Google Spreadsheets". If you click on that link, the file will be imported to Google Docs & Spreadsheets and you can start editing it. This is useful if you don't have Microsoft Office, OpenOffice or other office suite. You can also invite the person who sent your document to Google Docs & Spreadsheets, so you don't have to email the new version of the spreadsheet.
You can use the mail that contained the attachment as a bookmark: the next time you click on "Open in Google Spreadsheets", a dialog will offer to open the existing copy.
This is a good way to create an user base for Google Office, and we can expect to see other file types treated the same way in Gmail.
CSV files can also be imported this way, but there's no link in Gmail yet. If you want to try to import CSV files, you have to copy the link for "Download" and replace view=att with view=txatt.
Here's the message you see when you open an attachment:
This file is your Gmail attachment imported to Google Spreadsheets. Save your spreadsheets here, and get to them from any computer. Share with others, and everyone can edit online at the same time. No more emailing attachments back and forth!
Related:
Import Gmail attachments into Google Docs
Gmail has an MP3 player