Apple has stated that the next major update to its mobile operating system -- iOS 4.0 -- will be released today, June 21, 2010. So far only developers have been able to test the Golden Master ("GM") version of the 4.0 code, which is build number 8A293.
iPhone OS 3.0 was released at around 10:30 am PT/1:30pm ET, on June 17, 2009. So if it's any indication, we could see iOS 4.0 around the same time today. But that's only a theory.
While you're obsessively clicking on the "Check for Updates" button in iTunes there are a couple of things you can do to prepare.
For starters, make sure that you have iTunes 9.2 installed, as iOS 4 requires it. Second, it's also a good idea to plug your iPhone into iTunes and make sure that you have a full backup. This is important for the restoration process and so that you don't have to wait for it to back up once you're ready to install the update.
While iDevices get backed up automatically by syncing, you can confirm which backups are stored on your iTunes by going to Preferences > Devices. You can also force your device to backup by deleting the last backup in the "Device backups" list.