I set out to check if ignites rolled in 2.1, after a rather embarassing event of me typoing my own results and getting very confused in the process, Muphrid saved the day, dummy checked me, and here are the results:
Trial 1:
http://i142.photobucket.com/albums/r120/nkthrasher/Misc/ignite_1.jpg
First four crits totaled:
1254 + 1323 + 1186 + 1164 = 4927 * .4 to get total ignite damage = 1970.8 / 2 to get per tick damage = 985.4
Looking at the screen shot you see the tick for 985 at 00:27:41
Then at 00:27:42 we get another crit
1845 * .4 = 738 / 2 = 369 per tick coming from it. Since we've already used one tick of the earlier ignite, the remaining 985.4 damage is evenly split over the next two ticks from this ignite.
369 + (985.4 / 2) = 861.7
And in the screenshot we see a tick for 862 at 00:27:45, another one occured at :47 off the bottom of the shot.
Trial 2:
http://i142.photobucket.com/albums/r120/nkthrasher/Misc/ignite_2.jpg
http://i142.photobucket.com/albums/r120/nkthrasher/Misc/ignite_3.jpg
1206 + 1279 + 1284 = 3769 * .4 = 1507.6 / 2 = 753.8
Seen at 01:06:25 with the 754 tick.
2064 * .4 = 825.6 / 2 = 412.8 + (754 / 2) = 789.8
Seen at 01:06:29 and 01:06:31
Trial 3:
http://i142.photobucket.com/albums/r120/nkthrasher/Misc/ignite_4.jpg
1210 * .4 = 484 / 2 = 242
Seen at 01:26:26
1294 + 1352 = 2646 * .4 = 1058.4 / 2 = 529.2 + (242 / 2) = 650.2
Seen at 01:26:31 and 01:26:33
Trial 4:
http://i142.photobucket.com/albums/r120/nkthrasher/Misc/ignite_5.jpg
1434 + 1464 + 2528 = 2170.4 / 2 = 1085.2
Seen at 01:41:36 and 01:41:38
Conclusion: The model for how ignite works is that successive crits build on each other, aproximately two seconds after the last crit the tick for all preceeding crits occurs.
If a tick occurs, then another crit happens, the remaining ticks worth of damage from the previous ignite is evenly split over the new ignites two ticks.
Ignites do not roll, you do not gain any extra damage from them.
A side note, I encountered the simult-crit bug four times while testing this, it is very much so alive.
Edit:
http://img120.imageshack.us/img120/4512/yousuckattestingot3.jpg
It seems that through creative timing you can get a tick to occur and still distribute the tick over the following ignite. Using the screenshot from above (supplied by Wran):
1416 * .2 = 283.2
seen in that first ignite tick, the second tick goes off (assuming here, since Wran has no timestamps) at the same time the second crit does. Gets to do its 283 damage, then we add
1448 * .2 = 289.6 + (283.2 / 2) = 431.2 tick afterwards, which we see two of.
Regardless, conclusion stands, Ignites do not roll. You can cheat an extra tick out of it with lucky timing, but the model fo basic behavior stands.
So I talked to a GM about ignite...