You can see in the screenshot how Gmail suggests to open attached XLS (Microsoft Excel) files in Google Spreadsheets. I’m not sure if everyone can see this feature yet.
I guess we can expect much more of this cross-integration to be rolled out in the future... every little step can build Google’s strength in terms of lock-in*. E.g. when you are hooked on Gmail, using Google Spreadsheets suddenly becomes much more convenient, even when you might not prefer it over a competitor’s spreadsheets app; the same can work for each of their products, like web search, Google News, Google Talk and so on. Personally as user I don’t care right now ’cause I like how the usability of Google’s “office suite” is coming along, but I wonder if one day Google will face monopoly charges, pressuring them to always offer different vendors – like they already do for maps, as the screenshot below shows.