IRB (Institutional review board) : Do journals ever check? Does anyone ever check? I've got a project underway that involves human subjects, and I got IRB (Wikipedia Link) approval.  I'm thinking about my next project while working on this one.  I'd like to collect some exploratory data for the next one, and potentially make use of it in any resultant publications.  Nothing involves any real risks to respondents, so I feel that there are no ethical constraints binding me.  

But as a practical matter, will anyone ever check that any findings were pre-approved by an IRB to be looked-for?  What might the consequences of ignoring IRB be?

This is social science.
I have never heard of a journal checking for IRB approval. However, as @xLeitix notes, your university may care very much, since doing studies on humans without IRB approval may get it into legal issues.

And I personally know of one case where departmental colleagues initiated a scientific audit of a professor, requiring her to dig up all documentation (filled-in questionnaiers, raw data, ... and IRB approvals) for some studies they were concerned about. Everything checked out all right, but if the IRB approvals had not been there, the PI would have been in very deep doo-doo.

So: get your IRB approval, and make sure it's safe and sound, with scans and electronic backup. Keep it for the requisite number of years, even after the study has ended and been published. Inquire of your university how long that is. Given today's cost of storage, I would keep the scan around indefinitely.