Input lag from what I've found in pro-level reviews has two main causes:
1. having a scaler in the monitor (which would give single-input monitors with no scaler, such as the Samsung 305T and the Yamakasi Catleap, an advantage here; strangely, running at native to eliminate the need for scaling apparently doesn't help when the monitor has a scaler)
2. response time compensation (some monitors buffer a frame or two ahead to improve this)
There can also be additional input lag when using an interlaced video source, but that usually means an older video game console. For PC gaming, that doesn't get involved.
TFT Central uses a CRT for determining the relative input lag, the lowest they've found so far is 0.6ms average for the Dell U2312HM. That's an e-IPS panel with some ghosting, though. TFT Central's champion for lack of ghosting is the Samsung SM2233RZ, but that one has 15ms average input lag.
Also, a screen with 32 inches or higher diagonal is usually marketed as a TV instead of as a monitor, and it's hard to find input lag measurements for TVs.