Sony's Press Release Regarding the New Features
After downloading PS3 System Software Update 3.60, Plus members will have access to 150MB of space and will be able to store up to 1000 PS3 game saves.
To do so, simply highlight the game save you want to store in the [Saved Data Utility (PS3)], press the triangle button to bring up the options, select “copy” and then select “Online Storage” as the destination. In turn, you can access your Online Storage list via [Saved Data Utility (PS3)] when signed into PlayStation Network on a friend’s PS3 and download your stored game saves to carry on your game from where you left off.
If the developer has chosen to implement copy protection on their game saves, you can still copy those files to Online Storage but there will be a 24 hour period, after each save, before you can then download them on any PS3.
If your PlayStation Plus subscription expires then you will no longer be able to access Online Storage until you renew your subscription. However, any game saves stored will be kept for six months and you will be able to access them as before when you renew the subscription, as long as you do so within that six month period.
Hopefully that tells you everything you need to know about this new, much requested service. If you do need any more information, the official PlayStation forums have posted a guide, and ask us in the comments as always.