I've been working on SharePoint 2007 since just before Christmas, I've used it with InfoPath, Groove and around February I started using SharePoint Designer too. In the last 8-9 months I have learnt a hell of a lot just from messing around and trying to work out all the new features on SharePoint. I love SharePoint Designer as well it has virtually everything you could need to customise SharePoint, I especially love the Workflow wizard, it makes it so much easier to create a workflow rather than going through Visual Studios.
At the moment I am editing themes and another useful tool I've found in doing this is IE Developer Toolbar, it helps you to see which part of the CSS files you need to edit, and makes your job a whole lot easier too.
Anyway a few good SharePoint Blogs I've used are
http://www.sharepointblogs.com/helloitsliam/default.aspx (this is now at http://www.helloitsliam.com/default.aspx but all the old posts can be found on the first site)
There are loads more out there but these are a few of the ones I've liked the most.
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