Here is a couple of resources useful for MP authoring in OpsMgr2007. Its a completely different world when compared to MOM, and ppl do keep asking about this, so here it goes:
1. Authoring Console (now in RC version) has the Authoring Guide with it (you can get it from http://download.microsoft.com/download/7/4/d/74deff5e-449f-4a6b-91dd-ffbc117869a2/om2007_authconsole.exe). The system MPs (needed for authoring MPs basing on system MPs) you can get from an installation of OpsMgr 2007 server in %ProgramFiles%\System Center Operations Manager 2007.
2. www.authormps.com has some sample MPs and tutorials.
3. http://www.systemcenterforum.org/howto/ has some useful guides on authoring as well.
4. http://www.systemcenterforum.org/writing-and-debugging-opsmgr-2007-scripts-without-a-console/ shows how to test scripts in the console
5. Boris blog: http://blogs.msdn.com/boris_yanushpolsky/ goes into details on SCOM SDK.
6. Finally some XML editors: XMLSpy (http://www.altova.com/products/xmlspy/xml_editor.html), MS XML Notepad (http://www.microsoft.com/downloads/details.aspx?familyid=72d6aa49-787d-4118-ba5f-4f30fe913628), or just Visual Studio. Remember to load the schema to help you with checking validity of your MP: ManagementPackSchema.xsd from the Authoring Console install (by default in: %ProgramFiles%\System Center Operations Manager 2007\MP Schema)
[added] 7. Best practices on MP Authoring in a diagram: http://download.microsoft.com/download/f/a/7/fa73e146-ab8a-4002-9311-bfe69a570d28/BestPractices_Rule_Monitor_REV_110607.pdf
[added2] 8. Pete's guide to runtime scripting in OM07 - part 1: http://www.systemcenterforum.org/scripting-series-part-1-updating-mom-2005-runtime-scripts-for-opsmgr-and-sce/. More parts coming. Gr8 stuff, Peter!
[added3] 8. OpsMGr online SDK: http://msdn2.microsoft.com/en-us/library/bb437575.aspx
Hope you find it useful.