Preview at WWDC likely in June, followed by September launch.
Apple Preparing iOS 7.1.2 Update, Likely Addressing Email Encryption and iMessage Issues
Activity is still low, but that is typical for the early stages of update testing, particularly minor ones that may not require widespread internal or external testing. As a minor update, iOS 7.1.2 is likely to focus on bug fixes and other tweaks rather than major feature additions, and it likely will not even be put through a developer testing period before release.
Details on changes included in the update are currently unknown, but there are two issues likely to be addressed. One is a fix for an email attachment encryption issue disclosed several weeks ago. At the time the issue gained widespread publicity, Apple issued a statement indicating it was aware of the issue and that a fix would be included in a "future software update."
The second is a long-standing iMessage issue that has recently gained new attention. The issue, which results in lost messages for those switching away from the iPhone to another device and not having their phone numbers disassociated from iMessage, has been worsened recently by server problems. Just today, Apple reported those server problems have been fixed and that it will be including "an additional bug fix in a future software update".
For both of these issues, iOS 7.1.2 would be the first iOS update since Apple's promise of fixes, so it's reasonable to assume those problems will be addressed in the forthcoming update.
While Apple is expected to preview iOS 8 at its June 2 keynote and begin making builds available to developers, a public release of iOS 8 is not expected until around the September timeframe, in line with the release of new iPhone hardware. Apple has on occasion continued to release updates to iOS after it has begun developer testing of the next major version, but these updates have historically been specialized ones targeting significant performance or security issues rather than broader sets of improvements.
Heading into WWDC, traffic from devices on Apple's network identifying themselves as running iOS 8 has been rising fairly steadily in recent months, while OS X 10.10 traffic has been fairly stable.