Besides release notes which can contain information the other ways that I know of are:
1- Knowledge Base articles - An example is an article that just came out yesterday on ScreenOS and NSM Schemas
http://kb.juniper.net/index?page=content&id=KB12877&actp=search&searchid=1278447954098
2- Juniper Technical Bulletins - this is a great service - you can get emails letting you know when things like new schemas are published. From the support page under "Technical Bulletins" you can select the option to read the bulletins or to get email alerts. This will give you up to date information. So I just got bulletin #2010-07-860 on 7/1 announcing support for SA Version 7.0R1 in schema update #139.
Hope that helps you!