Players suffocated to death many many times crossing nether portal from either side. From first person perspective, sometime I died immediately due to suffocation. If I move quickly as soon as the loading screen is over, I may (10%-20% chance, with iron armor) survive. Form 3rd person perspective, the player was suffocating while standing in the portal (His screen was still showing "downloading terrain"). There isn't any block around the portals. Moving the portal and changing the size (to 3x3) does not help. It seems like the portal (purple thing) are solid blocks at receiving side.