Protein Wisdom has an article discussing the Iranian heavy-water production facilities, in which a reader e-mails Jeff discussing the potential use of the heavy-water reactor at Arak to produce tritium for fusion weapons. While the reader's comments aren't incorrect, there is a little bit more to it than it might appear at first glance.
I'm not certain about the contention regarding the use of the heavy water reactor as a means to produce thermonuclear weapons, as the more likely use appears to be to produce plutonium for fissile weapons small enough to be launched with current Iranian IRBMs.
Iran wishes to develop a nuclear weapon small enough to launch on top of its Shahab 3 or 4 missiles, it will most probably be an implosion device with a plutonium (Pu) core. The only way to acquire that is through reprocessing irradiated fuel. Bushehr is a light water reactor that has received much international attention and most probably will continue to be closely scrutinized, making it difficult to clandestinely remove its spent fuel for reprocessing. Even if the IR-40 has just as much attention, the Iranians would have a better chance of removing irradiated fuel or irradiating natural uranium targets for Pu production in this reactor. . (Link)
Indeed, a heavy water reactor is among the most dangerous in existence from a proliferation perspective. One reason is that the low neutron cross section of heavy water facilitates a high number of U238 (uranium-238 isotope) atoms to absorb neutrons, resulting in the production of a greater quantity and better quality of plutonium product. (Link)
and
Once fully operational, the Arak reactor can produce about nine kilograms of weapon-grade plutonium each year, or enough for about two nuclear weapons each year. (Link)
Generally speaking a heavy water reactor is viewed more seriously as it provides a way to produce plutonium from natural uranium without going through the uranium enrichment process. It should be noted, however, that a reactor such as the one at Arak can certainly produce tritium.
Tritium is important, even when not used in fusion devices, as it allows for the construction of the more complex boosted fission devices, which reduce the amount of material needed, thus allowing for smaller bombs, as well as making them essentially impervious to pre-detonation.
It is possible to use tritium in a staged radiation implosion device, such as a fission-fusion-fission, or fission-fusion-fusion, or similar designs (which are pretty much what are what people are describing when they talk about thermonuclear weapons or hydrogen bombs). However, tritium is sort of a pain to work with for various reasons, and most often you'll see something like Lithium deuteride used instead as it is less costly to be produced and when used in a design then forms tritium which is then used in the deuterium-tritium stage.
However, these sorts of designs are much more complex and really do need testing validation before they make for viable weapons. So, overall, the primary concern with Arak is the manufacture of plutonium for weaponizable fission devices. The secondary concern would be the use of tritium or other isotopes in a boosted fission design, although these are more complex, the size and yield advantage would create superior weapons. The least significant concern at this stage would be the development of fusion devices.