Sorry for my tardiness, I have not been visiting these forums much lately. So:
1. Why % for accessability?
Because if you use px, then users of internet explorer cannot resize your text. Not everyone has perfect eyesight, so you need to let them be able to resize your text, else they can't read it
4. Regading the "tantek celic box model hack" - due to a post I read somewhere I've developed a fear of "hacks" as they may present unforseen cross-browser compatability issues. How does one know when a hack is good to use or when it just "duct tapes" the problem?
Good, too many people rely on hacks too much. The problem is that most browser hacks use one (typically a parsing) bug to correct a (typically formatting/displays) bug. The problem occurs when the next version of the browser comes out. If both bugs are still present, then you are ok. If both are fixed then you are ok, but what if only ONE of the two bugs are fixed?
The tantek Celic hack IMO is safe. IE 5 has both bugs, but IE6 has neither. I would consider it safe to assume that future versions of IE will have neither bug. At the moment this is the ONLY browser tricking hack that I am happy about using.
2. display:inline; - use that in ADDITION to display:block; ?
Just display:inline on it's own. Yup it's a hack and I would avoid it where possible. However, it's relatively safe, the worse that could happen is that future versions of IE might display it the way it displays in IE6 now

3. Talk to me about xhtml - please?
If you know html, take a half hour (thats all it will take you) to read the following, as w3schools can explain far better than I can:
http://www.w3schools.com/xhtml/default.asp5. What's the difference between ".cssStyle" and "#cssStyle" ? is it a preference thing? The tuts I've been learning from seem to vary on which format they use - reason?
Yeah, what Sam said. Personaly, I use the ID's for my structural divs. If I have a nav column and a content column, the chances I will not never multiple instances. I rarely use classes at all, and tend to group elements using the DOM, there is a good article by Eric Meyer on the subject. I'll dig it out when I get a moment.
6. Please recommend some CSS resources if you know any you like.
My linky bits page, it's a little slim at the moment (I always forget to bookmark stuff so I have to search for it again later lol) but there are some handy sites listed:
http://www.caffeinefuelled.net/resources/
I'm trying to collect as much info as I can on my site, although maintenance has been neglected for the past month or so, for personal reasons.