It's unlikely that I'll ever have more than 1GB of email. However, I do have more than 1GB of photos (Picasa) and more than 1GB of files (Google Docs). Is there any way to use the storage from Gmail in these other services? Or, to possibly answer my own question, is Google just marketing the available storage of Gmail by banking on the fact that nobody is going to use that much (whereas it would be much easier for people to fill up their Picasa accounts)?
The only time that storage is shared between the other Google products is when additional space is purchased: https://www.google.com/accounts/purchasestorage
In the Google Picasa help section about Google Storage they say that "Free storage space is specific to each product. Picasa Web Albums offers 1 GB of storage for photos and videos only. Gmail provides 7+ GB (and counting) which is reserved just for Gmail messages. Docs gives you 1 GB for everything: all of your docs, spreadsheets, presentations, and uploaded files. Free storage from one product cannot be used by or transferred to another product."
-- EDIT --
As Al Everett pointed out the storage is now combined between Google's properties.
Google has just announced that Gmail, Drive, and Google+ Photos will now share 15GB of free storage.
With this new combined storage space, you won’t have to worry about how much you’re storing and where. For example, maybe you’re a heavy Gmail user but light on photos, or perhaps you were bumping up against your Drive storage limit but were only using 2 GB in Gmail. Now it doesn’t matter, because you can use your storage the way you want.