What is the army in Joel 2? I've always heard that the army described in Joel 2 was some kind of end-time army, but reading about in the NLT (as in the link) the chapter subtitle is "Locusts Invade like an Army". Is there a Biblical basis for either position?
There is very similar imagery in Revelation to particular verses in Joel. A comparison is shown below:

Joel 2:4-5 (NIV)
4 They have the appearance of horses;
they gallop along like cavalry.
5 With a noise like that of chariots
they leap over the mountaintops,
like a crackling fire consuming stubble,
like a mighty army drawn up for battle.
Revelation 9:7-8 (NIV)
7 The locusts looked like horses prepared for battle. On their heads they wore something like crowns of gold, and their faces resembled human faces. 8 Their hair was like women’s hair, and their teeth were like lions’ teeth. 9 They had breastplates like breastplates of iron, and the sound of their wings was like the thundering of many horses and chariots rushing into battle.

So, there is support for the idea that Joel's Army is an end-times army. But wait! Shortly after that bit in Joel, there's these few verses:

Joel 2:25-26 (NIV)
25 “I will repay you for the years the locusts have eaten—
the great locust and the young locust,
the other locusts and the locust swarm[b]—
my great army that I sent among you.
26 You will have plenty to eat, until you are full,
and you will praise the name of the LORD your God,
who has worked wonders for you;
never again will my people be shamed.

This sounds more literal, and as such, it makes it seem like the earlier passages may have been metaphors. However, immediately after that, it's back into end-timey talk:

Joel 2:28-32 (NIV)
28 “And afterward,
I will pour out my Spirit on all people.
Your sons and daughters will prophesy,
your old men will dream dreams,
your young men will see visions.
[...]

As it doesn't really make sense that Joel/God would insert a short literal passage in a chapter of symbolism, I'm going to say that Joel's Army appears to be an end-times army.