Neptune’s axis is tilted similarly to Earth’s, but because Neptune has an orbital period of 165 years, its seasons are considerably longer than ours and thus have required many years of careful observation to detect.
See the official press release for a great summary, and check out the brilliant photos and the 13MB QuickTime movie. You can also read additional details and see a couple of graphs showing the seasons on the UW-Madison SSEC’s web site (note its dateline is a mistake; it should read 2003, not 2002).
Read the media coverage on Reuters or in this brief article in Scientific American. And if you want to learn all about Neptune’s seasons, you can watch two one-hour lectures by Dr. Sromovsky on the Atmospheric Dynamics of Neptune, which he gave on April 2 (viewable with RealOne, QuickTime, or Windows Media Player).