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In which case, sure, if I have specific phrases that I'm targetting, then I use those actual phrases in the tag. It does get you into a situation though where if you have, say, 6 variants of the phrase, then your tag is going to look spammy for 'widgets', eg:
...large blue widgets, large red widgets, medium blue widgets, medium purple widgets, etc...
In that case I use the three main phrases and sacrifice the rest of the phrases in favour of using a word in the keywords tag a maximum of three times, eg:
large blue widgets, red widgets, fuzzy widgets, green, grey, purple... etc...
As far as separators go, I actually don't bother to use commas in my tags anymore.. just a space between the words.
At the end of the day, however, I give very little attention to the meta keywords tag as it is ignored by so many major engines/dirs these days due to spamming. I still think it is worth having just in terms of getting a small advantage with those smaller engines and dirs that do use it... but it's about bottom of my development priority list.
It is rainy day/hangover work at best.
Also, I forgot to note above.. I usually only bother with having about ten words or so as a maximum in the tag. Think in terms of using the tag to emphasis your real KEY words.
Interesting point on the body tag. Although I am careful about getting keywords high up within the body tags, I hadn't given much thought to the value of getting the whole body raised up in the code. I probably use a small enough <head> for it not to be an issue.. but something to keen in mind for sure.