Our vendor-supported server's O/S only supports PHP 5.1.6, which limits us to ZF 1.6. The current version of Zend Framework requires a higher version of PHP.
We're struggling to decide whether to adopt ZF because of this incompatibility. Is it feasible to develop (indefinitely) in these older versions of ZF or should we hold off?
Features, security, bugs, etc. Is this a path we don't want to go down or are these older versions perfectly usable in a production environment?
What a pity. There were lots of structural changes implemented in 1.7 that may not necessarily break backwards compatibility (I am not sure), but certainly changed some structural stuff.. for instance, the new Zend_Loader_Autoloader, and some stuff with bootstraps.
I believe Zend releases security updates for most things 1.x (see the svn repository).
You can develop in them as long as you want but 1.6 is, I think, around 2 years old. There have been security fixes that you would probably want to take advantage of and a LOT of new features. As with any software it's usually a good idea to be relatively close to the most current version.
Since you say PHP 5.1.6 I am assuming CentOS or RHEL. If so, you could install Zend Server or Zend Server CE (free) and be right up to the most current version of PHP.
Bug fixes in ZF are released in newer version of ZF and they are not "backported" to earlier releases.
PHP 5.1.6 was released more that 3 and a half years ago. Zend Framework 1.6 is also ancient. If you need vendor support you should go with Kevins advice and use Zend Server. If even that is not a possibility I would seriously think about changing your vendor provider if possible.
At this point in time, ZF isn't quite as mature and stable as something like ASP.NET (where many companies still use 2.0, when 4.0 was just released).
I can't say I would recommend developing exclusively with ZF 1.6 until you know you'll have a clear path to upgrade down the road.
You may want to investigate frameworks whose current versions support PHP 5.1.6, or switch vendors.
check http://abreakorcontinue.blogspot.com/2010/05/zend-framework-1104-php-versions.html for the minimum php version for each Zend Framework component for last version: 1.10.4