Engadget reports that Apple has sent out invitations for an event to be held on April 8. The focus of the event will be on iPhone OS 4.0. This shouldn’t come as too much of a surprise, Apple always seems to announce their next big iPhone OS update around this time of year. But, they do seem to be cutting it a little close with the invites.

The event will be held at Apple’s campus at 10AM PT.

There have been several rumors about iPhone OS 4.0, the most interesting of which are of a unified email inbox and an Expose-like interface for application switching (in other words, multi-tasking).

This is probably the best part about owning an iPhone OS device — it’s very likely that even if you purchased the first generation iPhone on June 29, 2007, you’ll still be able to update to iPhone OS 4.0. Every iPhone, iPod touch, and iPad is capable of updating to the latest software version, it’s a little bit tougher with other handset operating systems.