Comparisons to the Tet Offensive so far have been pretty far off.
ONE
Direct comparison is virtually pointless. The Vietnam conflict, and guerilla forces were the evolution of many decades of direct warfare. The guerilla forces and organisations had decades of fighting against Japanese, and French rule, by which time of the US intervention, they were well versed and possessed of mature systems from which to adapt and offend.
By comparison, even if real widespread and coordinated resistance exists in Iraq, it certainly hasn't had the comparable time period, and outside support - from China and the Soviet Union - for arms and food.
There is also the point that the region lacks the depth of cover and rough terrain that make infantry forces dominant - though this is not true in the cities, these cities can be isolated more completely than their Vietnam equivalent ever could - this providing a limit to the possible degree of coordination that was simply not possible to eliminate in Vietnam.
TWO
With regard to statements, such as, 'The Viet Cong ceased to exist after this offensive and the North suffered extensive losses' as evidence of failure on the revolutionary forces' part.
In reality, the Viet Cong did not cease to exist - though casualty has been widely estimated at around 2/3-3/4 and the more regular forces approximately 1/2, such losses were still far less than inflicted. Revolutionary forces spent the next 2 years recruiting and, in greater part, rearming for more modern warfare - as during this period the first armoured units entered from North Vietnam to conduct extended operations. The Vietcong itself went on to make up its number and enlarge, though it's operations were now directed predominantly toward political warfare. Casualties also served to eliminate larger portions of the existing and longstanding revolutionary powerstructure, increasing the degree of control the part thereby exerted in the south. Losses to allied forces were numerically higher than that of the revolutionary.
The offensive gained massive political victory, especially locally, showing that the ARVN was almost completely incapable of protecting it's territory, and saw revolutionary control of rural areas almost complete, especially due to the removal of SV and US forces to defend the cities.
Political ramifications as to the USA itself were obvious, as this was a point seeing dramatic increase in unrest over the war and subsequently increasing desperation to withdraw from it.
The only aspect of defeat that could be the case is in the lack of popular uprising from the cities. Though virtually all revolution assets in urban areas were killed/eliminated, popular uprising was not the dominant goal of the offensive, but, rather, one of a number.
It is certainly not the case that the offensive was a complete success, but to call it defeat is quite incorrect.
TET OFFENSIVE IN IRAQ
I think would be unlikely, very difficult, and virtually impossible.
1. The nature of the terrain is different between cities, and allows greater isolation, therefore making coordination more problematic.
2. Resistance completely lacks the degree of experience.
3. Lacks mass support from outside - from nations of similar resource to the USSR and China.