How do I let go of ED-E?
Solution 1:
If you are on the PC version, open the console (which is essentially the same as Fallout 3's console) and set ED-E as your target in the console - using the prid command - and moveto him to you. I lost some of my companions at one point and doing this got them all back.
Console commands:
prid 001732d1
moveto player
The refid will be different if you have upgraded ED-E. You can see all his refids here.
Type the commands into the console, save the game, then exit and restart Fallout New Vegas - Steam achievements will be enabled again and ED-E will be with you.