I tried to only include subjective statements in the first post. This will be my personal feelings regarding HtL and Parry
For the last 6 or so weeks of WotLK I imagine most tanks are in the 20%+ range of Parry, likely closer to 25% (unless you've reforged it to Mastery). If so, I would not think that seeking additional Parry for HtL up time it worth it. If you have significantly lower Parry chance (15% or so) the gains are pretty good, as getting 60 or so Parry Rating gives you about 8 secs of HtL uptime.
I'm not sure what levels of Avoidance tanks are at on the Cata Beta, but if they are in the lower numbers (sub 15%), it may very well be beneficial to trade some Dodge for Parry for increased HtL uptime. I have a feeling past 15% and you'd need to figure out if additional HtL uptime is worth the avoidance loss from changing Dodge into a lower amount of Parry.
I was also starting to do the math regarding "regearing" for HtL uptime versus gearing for straight threat (Crit mostly). I'm not done yet, but it seems that until at least about 20% Parry chance, you get more DPS/threat "regearing" for Parry versus regearing for Crit (due to HtL's +10% crit). It's slightly harder comparing regearing for Parry (and HtL) versus gearing for Mastery, as the HtL bonus is not quite the same thing as Mastery. I have a feeling the defense side of it will likely be a "aim for balance" strategy.
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