September seems more likely every week.
You can't throw a digital stick right now without hitting at least a couple of iPhone 5 rumors. We rounded up the bulk of them here but new ones crop up every day. Like this rumor from an iPhone manufacturing equipment supplier. This source reports that Apple's orders have been "several months later than usual". This adds more support to those rumors of a September iPhone 5 launch.
There are a few reasons to believe this rumor:
1. Apple's launches are not set in stone, year-to-year. The iPad 2 launched earlier in the year than its predecessor. And the iPhone's launch date has also moved around quite a lot. The 3G hit in July, while the other three iterations all launched near the end of June.
2. As noted by AppleInsider, the main impact of any new iPhone launch occurs in the September quarter. Launching in September would therefore be unlikely to have a substantial impact on Apple's bottom line.
3. The combination of a horrific earthquake and today's Foxconn factory explosion may very well have caused production shortages for Apple. The new iPhone's anticipated 4" display is another possible cause for the delay, due to touchscreen shortages.
That said, we're still a few weeks short of June. All rumors of a delay should be taken with a grain of salt for at least a few more weeks. If we haven't heard anything from Apple by the end of June, a September launch will be all but confirmed.