I have various calendars from iCloud and Google and it's very often that an event will be deleted from Google calendar but the iPhone will still show it. The same calendar is on iCal on my Mac and it shows it correctly there. If it doesn't, I can ⌘ + R or ⇧ + ⌘ + R to update it.
Maybe I'm missing the place or configuration to do the same on the iPhone calendar application.
Best Answer
EDIT: This has been fixed in recent versions of iOS. Please see @LukeWilliams answer which describes simply pulling to refresh the calendars in the Calendars list.
For deprecated versions of iOS: As of iOS 5 there is no super easy/obvious way to do this. What I have found that works, is opening Settings > Mail, Contacts, and Calendars > Tapping the email account you'd like to refresh > turn the calendar off > wait while it deletes it > turn the calendar back on.
sometimes by forcing a New Event, it triggers syncing.