Updated January 12th
YouTube is changing how Subscribers are being counted. These changes are intended to remove inactive and closed accounts from total subscriber counts as well as improve techniques for preventing artificial inflation of subscriber counts.
The changes go into effect January 12, 2012, and are being implemented to, "ensure that all subscriber counts are as accurate as possible". The official blog post says that subscriptions generated from malicious sources (such as automated systems or scripts) will not be added to subscriber counts.
(There has already been a bit of confusion as to the definition of an 'inactive' account. This post on the YouTube Help Forum helps clear that up a bit. 'Inactive' does not mean an open account that someone hasn't used for a while.)
Even with this change, any accounts that are subscribed to a user will still receive video updates from said user.
Update: Due to synchronization issues with YouTube Analytics, the subscriber count change has been postponed to a currently unannounced date. The system to prevent automated subscriptions is still reported to be on schedule.