Figured I would reply to Tom's post since I also had something to say about Dawn's "rule of thumb" for gemming:

Although it seems to work out most of the time, it won't always. Shadow priest are better off knowing how much value each stat holds. http://www.wowhead.com/item=47051 for instance, the Socket Bonus of +7 Spell Power with 2 Purified Dreadstones would be worse than just gemming with 2 Runed Cardinal Rubies.

It also doesn't address what to do if the socket bonus is some other stat. I guess it's fine if you want to follow this gemming advice, but I like to know that I'm getting the most PP for my buck if I'm gonna be spending hundreds on gemming gear.

http://www.shadowpriest.com/viewtopic.php?p=201757#p201765

Quite.

Be careful about falling prey to socket bonuses. In general you want to put +23 spellpower gems in everything, regardless of color/bonus. There are two exceptions:
1. You need 2 blue gems to activate your meta. Try to slot 2 purple Purified Dreadstones in appropriate slots.
2. The socket bonus is at least 6 PP (pseudospellpower, all DPS-effecting stats for SPriests can be rated in terms of how much spellpower it would take to equal their effect on your DPS), then it is worth going for. Otherwise, it will likely result in a DPS loss to get it.

I'd encourage anyone thinking of rolling Shadow to check out Shadowpriest.com's stickies, especially:
1. the 'best raid gear' thread: http://www.shadowpriest.com/viewtopic.php?f=62&t=24134
2. the 'PvE specs' thread: http://shadowpriest.com/viewtopic.php?f=63&t=25652&start=0
3. the '80 PvE FAQ..' thread: http://www.shadowpriest.com/viewtopic.php?f=47&t=23007
In general all the original posts in the threads above are rather clear, direct, and explain the how and why behind the decisions they present. While to the best of my knowledge Fox hasn't said anything wrong, I'd advise against trying to extract guidance from his... writing.