Depends on the ending but the general idea is that the energy in the relays is dispersed to do w/e color BS you choose so they don't kersplode entire systems. Either way anyone that went to earth is mostly screwed Bioware forums full of people theorycrafting and mathing travel times etc. Based on speeds quoted from characters in game apparently it takes 27 years to go from one end to the galaxy to the other thru FTL. Turians and Salarian space are closest to earth. So not even factoring need for fuel anyone that went is still kinda screwed since Turian/Quarian have food issues, Krogan dont have their own ships since they demilitarized, Salarians traveling home be good portion of their lifespan.
Blowing up relays seems like a dumb plot device to skip story of next game ahead X years till races recover rebuild and any direct ties to current ME are long gone or dead(maybe Liara or wrex might still be around).
In related ME news this pic been floating around on Bioware forums
http://i.imgur.com/xjXdX.png
The single player DLC is probally like 99% bullshit but the new multiplayer seems to be real.
Souj ~ Its glitched and doesn't show up for alot of people but its still there its just a item you pick up from scanning the planet.