As noted by Plaffo.com, WMPowerUser, and a number of other sites Microsoft recently published updated information on support dates for the Windows Phone platform. Microsoft says it will make updates (i.e. security and bug fixes) available for each version of Windows Phone for 18 months after the lifecycle start date (roughly a date shortly after which each version first becomes available).

For Windows Phone 8 the lifecycle start date was 14th December 2012, giving a support end date of July 8th 2014. For Windows Phone 7.8 the lifecycle start was February 9th 2013, giving a support end date of 9th September 2014. This is in line with the previous support date for earlier versions of Windows Phone. However, it's worth noting that Windows Phone 8, like Windows Phone 7 and 7.5, will almost certainly recieve an update (e.g. Windows Phone 8.5), which would extend the support date by another 18 months.

Here's how Microsoft detail the timing of the lifecycle support period:

Microsoft will make updates available for the Operating System on your phone, including security updates, for a period of 18 months after the lifecycle start date. Distribution of the updates may be controlled by the mobile operator or the phone manufacturer from which you purchased your phone. Update availability will also vary by country, region, and hardware capabilities.

It's important to note that these support dates do not have a direct bearing of whether a major version update will be released for a given platform version. Furthermore, they only relate to the support for the software provided by Microsoft (i.e. they do not have an impact on manufacturer support).

It is unlikely that devices running Windows Phone 7.8 will recieve a major update, but the support dates detailed above mean it will be supported until September next year and therefore may receieve small security or bug fix related updates (e.g. the recent update to fix the Live Tile update issue).

It is very likely that devices currently running Windows Phone 8 will receieve an update. At the preview launch of Windows Phone 8 Microsoft made it clear that it anticipated all devices would receive at least one major version update after their initial announcement (i.e. Windows Phone 8 devices would get the next major update).

At the Windows Phone 8 preview event Microsoft also mentioned that it was planning to roll out a program that will give early adopters access to software updates in advance of general availability. We asked a Microsoft's spokesperson about this early adopter access program at MWC and it's still very much alive, although there was nothing further to announce on details or availability.