jQuery will be included in the WRT development kit which is used to create widgets for the S60 platform. Standardising around jQuery should encourage widget developers to use the built in library rather than including one themselves. It also likely that a thorough testing program will take place to ensure 100% compatibility between the library and the S60 browser.
Here's an extract from the blog post:
Nokia is looking to use jQuery to develop applications for their WebKit-based Web Run-Time. The run-time is a stripped-down browser rendering engine that allows for easy, but powerful, application development. This means that jQuery will be distributed on all Nokia phones that include the web run-time.