Opposition to Tisha B'Av in the present time tends to fall into two major camps: the anti-Zionist and the Zionist (though some people oppose it for both reasons). The first is exemplified by the Pittsburgh Platform (1885):
We recognize, in the modern era of universal culture of heart and intellect, the approaching of the realization of Israel's great Messianic hope for the establishment of the kingdom of truth, justice, and peace among all men. We consider ourselves no longer a nation, but a religious community, and therefore expect neither a return to Palestine, nor a sacrificial worship under the sons of Aaron, nor the restoration of any of the laws concerning the Jewish state.
In other words, since we don't want to go back to Israel or rebuild the Temple, why mourn the Diaspora or the destruction of the Temple?
The second (Zionist) approach is that now that the State of Israel has been established and millions of Jews have returned to Eretz Yisrael, the reasons for mourning no longer apply and therefore the fast of Tisha B'Av should be eliminated or at least diminished.
Both of these approaches have in common the idea that modern developments have brought us qualitatively closer to the messianic era. I do not share their optimism. Looking around in 2005, it is clear that neither the "universal culture of heart and intellect" nor the State of Israel has succeeded in bringing redemption. And this recognition of how far we still are from "the establishment of the kingdom of truth, justice, and peace among all [humans]" is the reason that I still observe Tisha B'Av.
Do I want to see the literal reinstatement of the Temple service like these folks? Nah. (Though I wouldn't mind the "house of prayer for all nations" that we read about this afternoon.) But so what? Our yearnings for the rebuilding of the Temple don't have to be literal; they can be all about the aforementioned kingdom of truth, justice, and peace. It surprises me that communities that are willing to understand our national stories about the past in metaphorical and homiletical ways (e.g. creation; the exodus from Egypt; heck, most of Tanakh) are reluctant to do so with our stories about the future. This is why I say "na'aseh v'nakriv" (future tense) in musaf, and not "asu v'hikrivu" (past tense): if it's viewed historically as a stage in our development, then it becomes actually all about the dead lambs, whereas if it is in the future tense, then it is clear to me that it's all figurative.
But that's the thing: once the yearning for a rebuilt Jerusalem becomes figurative, the rebuilding of the physical Jerusalem no longer has any influence on that yearning, except insofar as it brings us closer to "truth, justice, and peace". (And I do think the existence of the State of Israel is a good thing, but it's insufficient.) Thus in mincha today, I said "ha'ir ha'aveilah v'hachareivah" ("the city in mourning and in ruins"), and not one of the updated versions in Siddur Sim Shalom and elsewhere that say "the city that was once in ruins". (Gates of Prayer maintains the traditional text in Hebrew, but renders the English in the past tense. I don't do it that way either.) There is so much desolation in Jerusalem and the rest of the world that the mere physical reconstruction of Jerusalem cannot bring it out of its state of mourning.
So I observe Tisha B'Av because modernist/Zionist triumphalism has given way to postmodernist pessimism, or something like that.
The rabbis were wise in folding all of our tragedies into a few fast days, because if we were to mourn each historical tragedy separately, we would never stop mourning.
I don't even think so much on the fast days about historical suffering (other than the archetypal kind), because there are more than enough examples of suffering in the present to keep me occupied.
Someone suggested that if the fasts are going to be about present-day suffering (rather than about Gedaliah ben Ahikam himself, etc.), then there should be a specific theme for each one. I agree, and I propose (for American Jews) devoting the three minor fasts associated with Tisha B'Av to the US, Israel, and the rest of the world. Matching up each fast (17 Tammuz, 3 Tishrei, and 10 Tevet) with one of those themes is left as an exercise to the reader (please post your ideas in the comments, with an explanation). Bonus points for a tie-in with the Fast of Esther.
According to Zechariah, these four fasts will be observed as days of joy in the future. May the next Tisha B'Av be one when celebration is justified, bimheirah v'yameinu amen.