Considering the time, effort and cost that would go into re-specializing, you might be better off staying with what you have earned.
The other thing to consider is that if you re-specialized, you would have to give up your armorsmithing breastplate. In addition, with Wrath of the Lich King's release only six weeks away (November 13th), I would not recommend this as the prime time to switch.
So what does Wrath have in store for blacksmiths?
Unfortunately, it seems that items that require a spec to forge will still require that specialization to wield or wear, although we do have a preview of some of the recipes currently in the Beta that blacksmithing and its specs will have to offer.
You will also be able to add sockets to items, some of which can only be used by you, and others that can be applied for other players in exchange for gold or /kisses, and it is definitely one reason to stick with the blacksmithing path.
If you still feel that the grass may be greener for the other specialization, here is a breakdown of how the various items compare to their non-crafted counterparts:
[Bulwark of the Ancient Kings] is a nice chest piece for fury warriors, but it could also be used to tank up to Karazhan as long as you can reach the appropriate levels of defense and mitigation through other means.
This crafted piece is actually comparable to Tier 6 (Black Temple, Mount Hyjal) plate chest pieces, believe it or not.
[Onslaught Breastplate], a Tier 6 piece that can be purchased using a token that may drop after killing Illidan, is a warrior-only but similar piece, although the itemization is a bit different because of the "use" effect on the armorsmith piece.
This would likely be the first upgrade to that breastplate by a fury warrior, given that the Tier 5 equivalent, [Destroyer Breastplate] has inferior stamina, hit rating, critical strike rating, and depends on having at least two pieces from the set to begin to compete with the crafted piece's use effect.
In fact, if you prefer critical strike rating to agility, you might not equip the Tier 6 piece either until you had a second piece from that set in order to add the extra incentive, although you would probably do more damage with it anyway because its effect is not limited to use.
Unfortunately for tanks, the armorsmith breastplate would be replaced in Karazhan by the [Panzar'Thar Breastplate], whether you are a warrior or a paladin.
If you are a holy or protection paladin, or protection warrior, it seems that armorsmithing itself is not going to bring you any benefit, although of course there are non-specialization but bind on pick-up crafted items that still make being a blacksmith helpful.
If we were to compare the bulwark to the Tier 6 paladin piece for retribution spec, [Lightbringer Breastplate], you might be surprised that the two pieces are actually quite similar. The crafted piece has 16 less strength, but 4 more hit rating, 10 more critical strike rating, 8 more stamina, and the use effect.
The Lightbringer piece brings intellect to the table, so if you cannot muster enough intellect from your other pieces, you'll probably need to switch to this one. Although you should be compensating for the lack of intellect using gems if necessary, but once you can pick up this piece, you could free up your gem slots for something more beneficial to your DPS.
The other thing to note is that the two piece set bonus would also generate mana as a chance on hit sort of effect.
If you think that I have forgotten the arms warriors, I can assure you that I have not. Because arms is the preferred PvP spec and not as suited to PvE/raiding, most arms warriors are going to be sporting PvP gear purchased by running battlegrounds or arena. Of course, if you do not like to re-spec but you do raid on occasion, having the PvE breastplate is not a bad idea.
In fact, if you can get your hands on the arena season four piece, [Brutal Gladiator's Plate Chestpiece], you'd have a clean upgrade, even for PvE, with 158 more armor, 5 more strength, 20 more stamina, 4 more critical strike rating, and only 7 less hit rating. Your attacks would also ignore 84 of your target's armor, and of course, the resilience does not hurt.
The season three piece however is a bit more of a toss up when it comes to PvE. [Vengeful Gladiator's Plate Chestpiece] is the same item level as the crafted breastplate. Both have 1825 armor, but the PvP itemization takes some points away from your DPS and into survivability. With one less strength, 2 less critical strike rating, and 13 less hit rating, then you might slip down the charts if you cannot maintain the proper hit rating to avoid misses against raid bosses.
That being said, the extra 9 stamina, 24 resilience and the ability to ignore 84 armor when attacking could fill holes that your other gear does not, and it would seem to be a matter of personal preference.
Adding blue gems to the Bulwark piece:
As for maximizing the socket benefits from the crafted piece, fury and raiding arms warriors should consider using two [Smooth Dawnstones] and one [Jagged Talasite] for a bonus of +6 stamina and +20 critical strike rating.
Retribution paladins could consider adding three [Brilliant Dawnstones] for +24 intellect and ignore the socket bonus, or try for three [Bold Living Rubies] at +24 strength, also ignoring the socket bonus, or a mixture of the two. If you like the set bonus, use one of each and something like a [Soverien Nightseye] or a [Jagged Talasite].