How to setup a quota for the whole "Mysite" webapplication in sharepoint 2007?
Is it possible in SharePoint 2010?
Under my knowledge so you cannot set up the quota for an entire web app.. this is limited to site collection scope only.
You could do this indirectly by limiting the size of your content databases. Here's an example:
25 Mysites, each with a site collection quota of 2GB. Set your content database up pre-sized to 50GB with auto-grow disabled. Since you have control over the amount of content databases you are dictating how much space that web application can utilize.