My disk isn't exactly speedy (laptop, and not a solid state drive but it is 7200rpm), and initrd isn't taking that much time to load. systemd is reporting it as taking about 4.2 sec. What really needs to be fixed is user space, especially getting to a usable desktop fast. Gnome should've fixed this somewhat with their read-optimized settings file dconf, but it hasn't fixed it enough, IMHO. The lighter desktops won't have this problem, of course, but their solutions won't work for KDE/Gnome.
Of course, this is all assuming fast boots are significent enough that they warrant the engineering effort (I am, of course, not talking about embedded market).
Something that might be interesting to purse when btrfs matures enough is to actually rearrange the file layout after every boot (or, after X many boots, or when boots start taking too long, or whatever). As much a possible, the files needed to get to a fully functioning desktop are arranged in a linear manner starting from the earliest possible sector. Obviously the goal would be a continuous read, so it should max out the disk. Now, this wouldn't work in SSD (since they completely mask the physical layout, IIRC), but it would also not matter as much since they already provide very fast boots.
This wouldn't obviate the need to provide some tweaks in userspace.