At this writing Google is taking email addresses of those that want to try Google Spreadsheets, but it took less than a day to get my invitation when I registered. The wait may become longer as they get more users on board.
Google Spreadsheets does not offer the full range of features that are found in Microsoft Excel or Open Office Calc, but it is still a slick little program which could be very useful depending on your needs. The biggest advantage to Google Spreadsheets is the ability to easily publish the spreadsheets online for free. If you manage any kind of shared data, this may be the way to go.
The list of formulas available in Google Spreadsheets may be small compared to the big offline counterparts, but they offer more than enough flexibility for most common uses. In this first incarnation, Google Spreadsheets offers over 200 formulas.
You can save your spreadsheets online and you can download your saved spreadsheets if you wish. The spreadsheets can be saved in Microsoft Excel (xls) format or as a comma separated values (csv) file. You can load your current spreadsheets as long as they are fairly simple and not larger than the limited cell dimensions of Google Spreadsheets.