One of the most interesting feature in Office 365 is the ability to send out invites to people who do NOT have an Office 365 account. These people can be your customers, busniess partners etc.
Click here, the link that talks about how we can configure it.
Also now after the Service Update, released in November 2011, we can invite upto 1000 people. And we can change the inheritence and restrict external invitees from accessing all the content. We can restrict them from access a document library or document/item.
IMPORTANT : The number is not increased to 10,000 PAL.
For better planning to use Office 365 in every scenario, do look into Office 365 Advisor. This is one of the Office 365 tools.
NOTE : External invite can only be sent at the site collection level.