Well, it looks like we'll know sometime after E3. From the twitter thread:
Andy McNamara @GI_AndyMc
Since I've been asked a couple times, cover reveal is delayed a bit due to E3 but it's coming. 
Edit: After reading further in the Twitter thread, it seems like the devs are still trying to get the cover art nailed down. Hopefully the devs have been reading this thread and considered some of the various ideas we've talked about. To recap:
1) Put a helmet or a weapon on the cover with the logo and an alien landscape.
2) Have on PC on the cover looking at an alien landscape, no discernable gender.
3) Just one defined gender on the cover.
4) Both genders in "action" poses.
5) Both genders in relaxed poses, perhaps looking out towards the stars.
Please devs, if you are reading this, don't put the guy in an "action" pose and leave the gal standing there checking her
watch omni-tool with her back contorted to show off her chest. It looks silly and painful. The suggestion giving for an action pose to a lady PC is showing off a biotic field right before they are getting ready to use Warp on someone.