Can you light the spell grease on fire? If so, what are the effects of doing so? (How long does it burn, does it do damage to creatures standing in it / coated with it, etc.)
No, Grease (as per the spell), is not flammable.
There is already a spell that does this:
(Complete Mage, p. 108)
Level: Sorcerer 2, Wizard 2,
A slick, greasy liquid seeps from the targeted object, thoroughly coating it. This spell functions like grease (PHB 237), but the liquid is also highly flammable. If any fire damage is dealt within the area of the spell (or to the subject of the spell), the spell's area (or subject) bursts into flame. This effect deals 4d6 points of damage to anyone in the area (or holding the subject), but also ends the spell's duration. A successful Reflex save halves this damage. (emphasis mine)
They would not have created a level two spell that specifically calls out being flammable as the distinction if the level one spell were already flammable.
Grease puts out fires, and can be used to coat objects to prevent fire damage.
Since the grease has been conjured (is not an illusion) logic dictates it must have consistent material properties.
If the popular notion is that the grease is not flammable, then that (it’s fireproofness) is a useful property which can be utilized.