The Islamic Republic of Iran has cut ties with the British Museum over the Museum's decision to delay a loan of the so-called Cyrus Cylinder. See http://news.bbc.co.uk/2/hi/entertainment/arts_and_culture/8502654.stm
Diplomatic relations between the two countries have been particularly poor since Iran's stolen presidential election. Indeed, the Iranians have branded the UK the "Little Satan" [to distinguish it from the "Great Satan," i.e., the US] due to the UK's support for the pro-democracy demonstrators. See http://www.time.com/time/world/article/0,8599,1907066,00.html
In any event, I suspect the current Iranian regime wants to display the seal at least in part because it feels it will confer some sort of legitimacy upon it, as the guardian of the great Persian civilization. If so, this a bit of an odd throwback to the time of the Shah, who also tried to identify his own government with the glories of the past.
And this desire to display an artifact associated with the glories of ancient Persia is more than a bit ironic too. After the Iranian revolution, there was serious talk of bulldozing Persepolis, and even now this world heritage site is both neglected and in danger of damage due to pollution. See http://www.cais-soas.com/News/2002/May2002/10-05.htm
Under the circumstances, I cannot fault the British for holding onto the Cyrus Cylinder. The British have every right to decide when and whether to loan the seal.
And as was noted by Chuck Jones on the SAFE Corner blog (see http://safecorner.savingantiquities.org/2010/01/another-delay-for-cyrus-cylinder.html), this is certainly not a repatriation issue as the Cylinder was found in Iraq. Hence, the arguments usually marshaled for such claims simply do not apply in this particular case.