First of all: right now I am talking about the white screen for too long problem (for me, it was about 40 seconds). I also get the 5second blinking, but that doesn't bother me very much. I have a 2007 iMac with a custom-installed Intel X25-M 80GB G2 SSD. Finally, my goal is to have an iMac running solely W7 x64.
For me, a clean OSX-Bootcamp-Windows install always fixed the problem (white screen for 40sec instead of 15). Then it SEEMED to go wrong after I booted from a Norton Ghost 2003 recovery CD (holding down C when booting), altough I AM BY NO MEANS SURE IT IS THE TRUE CAUSE, it might have been some other coincidence. When I partitioned the disk using W7's installer, the problem also showed up. I could always "fix" te problem by holding down Alt at boot up then selecting the disk I want to boot from. This way there was no white screen for 40sec. (Try this trick.) It seemed to me that after booting once from the CD the iMac had a hard time deciding which disk to boot from, and reinstalling OSX somehow resets this behaviour in the EFI. (there is no way to directly access the EFI)
Right now, my problem appears to be fixed. Here's how I did it:
1. I booted the OSX DVD, then created a GUID/GPT partition system with two MS-DOS FAT partitions (the second one for data. the disk utility created a hidden small third one, of course)
2. Without installing OSX, I booted the W7 x64 DVD (Note: I had to create an ISO file from the W7 DVD, then fit it for the Mac according to this link:
http://sergiomcfly.blogspot.com/2008/04/select-cd-rom-boot-type-when-installing. html then burn it to an empty DVD. Otherwise I got the "Select CD-ROM Boot Type" error. This note has nothing to do with our current problem. Without this, Win7 wouldn't even install.)
3. I formatted the two partitions to NTFS using the W7 installer, then put Win7 to the first one.
4. I used viper's activation method.
Note: I always booted from the CD drive using the Alt method.
I can't tell you which one of the preceding maneuvers fixed my problem.
I have booted the Ghost 2003 CD multiple times since then, but the problem has not resurfaced. (BTW Ghost 2003 works perfectly, altough it is a PC-DOS based system and certainly doesn't know anything about GUID/GPT.)
An interesting note: right now, holding down C when booting SEEMS ineffective in making the system boot from CD. (I only tried it twice, though.) The only method to boot from the optical drive is to hold down Alt, then select the drive.