The WordPress development team has decided to hold back version 2.2 for at least a week or two from the original release date of April 23. One of the new features in WordPress 2.2 is built-in Tags support, which’ll eliminate the usage of external tag plugins that we all have been using e.g. UTW, Jerome's Keywords, Simple Tagging etc. Most probably, it’s the Tags support behind this delay.
A tag is a relevant keyword or term associated with or assigned to a piece of information (like picture, article, or video clip), thus describing the item and enabling keyword-based classification of information it is applied to.
Source:→ Matt's BlogWordPress, WordPress 2.2, Tag, Tags, Release, Releases, Postponed, Delayed