I do not understand your rationale.

Converting Frost Emblems to Primordials (I can only assume to AH them) may be a greater loss than allowing them to convert to Justice Points considering the rate of inflation from Vanilla to BC and then to WotLK. We'll have no idea until we find out how many Justice Points it takes to buy something useful in Cataclysm.

Buying Primordials outright (with gold) makes even less sense unless you are buying gear you will actually be using, as demand for Primordials should only fall at this point as interest in raiding ICC declines - there are only so many times I can run the same raid, after all - demand for items that require Primordials wanes, and ultimately Primordial Saronite becomes obsolete with the release of Cataclysm.

More confusing is that Emblems of Triumph and Frost will convert to Justice Points at the same rate; surely Emblems of Frost should be worth more than Triumph? This logic suggests it is more efficient to farm Heroics than to run current raid content in preparation for Cataclysm.

At this point the best advice is probably to clear out any items that are primarily for current WotLK raiding content - Primordials, Eternals, etc. - in preparation for the expansion, and wait and see (and hope) if the conversion rates get tweaked.