Google has recently enhanced Google Spreadsheets that are now decked with the ability of editing documents from mobile devices, including the iPhone and iPod touch as well.
According to the Official Google Docs Blog: “You can quickly view, edit, sort, and filter your spreadsheets on a variety of mobile devices including Android-powered devices like the T-Mobile G1, the iPhone and iPod touch, and the Nokia S60.”
Just log on to m.google.com/docs on your iPhone or iPod Touch, and in case if you are already using Google Apps, then you can log on to docs.google.com/a/your-domain and try this enhancement!