Link - ArcGIS 9.1 Service Pack 1
ESRI recommends that all ArcGIS 9.1 customers download and install this Service Pack, at their earliest convenience, to ensure the highest quality experience when working with ArcGIS 9.1.
Click here to read the issues addressed with ArcGIS 9.1 Service Pack 1. SP1 is available for ArcGIS Desktop, ArcGIS Engine Runtime, ArcGIS Engine Developer Kit, ArcReader and ArcGIS Server. SP1 for ArcGIS Desktop runs pretty large at over 60 MB so you might want to grab it before the download servers get swamped.
Update - Looks like ArcIMS and ArcSDE SP1 are “coming soon”.