Replying to Paul Guijt:
OK, I regretfully respect that. But could you please take care that a ticket is opened to have WordPress communicate with Wordpress.org and Wordpress.com (and any other site that is only IPv4) through IPv4 only?
This is a server configuration issue on the hosts end. As only IPv4 DNS records are returned, it's up to the hosts PHP configuration being configured correctly to connect over IPv4. If an IPv4 gateway isn't available and only IPv6 is enabled, then it would be expected that the HTTP requests would fail quite fast due to the underlying network not supporting it.
WordPress.org's API will be available over IPv6 some day when our hosting infrastructure supports it, so we're not going to lock it down to only occurring over IPv4.
There have been some bugs in PHP's CURL implementation in the past which causes IPv4 connections to fail on IPv6-only hosts even when a IPv4 gateway is present which can't be worked around reliably from within WordPress.