I'm not interested in using terms for their pejorative effectiveness, but rather their accuracy in designating something.
According to Wikipedia (the authority for everything now), "Fascism is a radical political ideology that combines elements of corporatism, authoritarianism, nationalism, militarism, anti-anarchism, anti-communism and anti-liberalism."
Because Fascism is a historical term that refers to the international threat fought during the Second World War, I think it misleading to use the term Islamo-fascism to refer to the current Islamic menace. I prefer instead to refer to the enemy as Islamism, highlighting the religious aspect of its radicalism.
While authoritarianism, militarism, anti-semitism, and anti-liberalism characterize the enemy we now face, several crucial features are missing that would identify them with the older enemy of half a century ago.
Most importantly, Islamism manifests no nationalistic element. While Islamists oppose the existence of an Israeli state, they do not organize their hopes around the destiny of any particular nation or ethnic group. The absence of a nationalistic cause disqualifies Islamism from being identified as fascistic.
Furthermore, Islamist states (like Iran and Libya) have been known to experiment with both communist and corporatist economic programs. While Hitler and Mussolini hated communism, Islamism does not define itself as the opposition of international communism, which has lost its impetus as a worldwide movement.
Given their current political alliances (formal or informal) with Cuba, North Korea, Venezuala, and the liberal left in western countries, Islamist groups show a willingness to ally themselves with communists or otherwise who opposes America's liberal-capitalist hegemony. At the risk of painting with too broad a brush, the left's nearly unanimous siding with the Palestinians against the Israelis, the presence of Iranian diplomats during North Korea's missiles tests in July, and this month's Cuban meeting of the Non-Aligned powers (led by prominent leftist and Islamist national leaders), all point to a convergence in interests between the two groups. So while naming this phenomenon Islamo-communism would be joining what is ideologically incoherent, the term Axis of Evil is an apt designation.
Concluding, Fascism and Islamism represent different ideologies interested in divergent objectives (e.g., racial purity vs. religious purity). While sharing similar features, they are also dissimilar, being threats that arose for distinct historical reasons. Islamism is therefore a better designation than Islamo-fascism. Islamists are currently alligned with the heirs of the communist legacy, but this alliance appears temporary and pragmatic in motivation, unless one has special apprehension of the spiritual powers that may be at work.