For the upcoming patch (4.0.1), Blizzard has changed the conversion rate for emblems of justice and frost:

0. Currency Conversion -- Justice Point Update 09/29/2010 08:53:02 AM PDT
We’ve revised the Justice Points conversion rate for Emblems of Triumph and Emblems of Frost. When the upcoming 4.0.1 patch goes live, these emblems will be converted into 11.58 Justice Points (up from the previously planned 2.75 Justice Points per emblem). Justice Points accrued above the soft cap of 4,000 will be converted to gold at a rate of 47.5 silver per point (down from 2 gold per point) when patch 4.0.3 goes live.

Prior to the patch, gear which you can currently acquire for 95 frost emblems, for example, will cost 1100 justice points after the patch for a ratio of 1100/95 = 11.579. I.e., frost emblems will convert at a "fair rate"; you don't have to worry about spending them on gear before the upcoming patch goes live.

With triumph emblems, a piece of gear which you can currently acquire for 75 triumph emblems, for example, will cost 1196 justice points for a ratio of 1196/75 = 15.95. Since this exceeds the 11.58 conversion rate, the same gear becomes more expensive after the patch. So if you are still acquiring tier 9 gear (lvl 245), you are better off getting it before the patch goes into affect. If, however, you only need tier 10 gear (lvl 264), you might consider farming triumph emblems before the patch since they convert at the same ratio as frost emblems, effectively making the tier 10 gear cheaper after the patch.

Of course, if you already have more emblems than you can possibly use, you could consider trading them in for Crusader Orbs (triumph) and/or Primordial Saronite (frost) and selling them at the AH. This is especially true if you have more than 345 emblems (345*11.58=3995 justice points) because of the 4000 justice point (soft) cap being implemented in 4.0.1. While you won't lose any justice points at the conversion, you will not be able to acquire any additional justice points (from quests, raids, etc.) until you spend your justice points down to below the 4000 cap. And, as of patch 4.0.3, any justice points above 4000 get converted to gold at a low rate of 47.5 silver per point. Since you can buy a Primordial Saronite for 266 Justice Points and sell it at the AH for around 400 gold, this is better than receiving only (266*.475=126.35) gold in the 4.0.3 conversion.

Note that this information is based on the currnt PTR for 4.0.1. Things may still change before the release.