@peterleinchen,

U-boot can (for now only be installed in the kernel partition. Meaning the kernel proper has to be piggybacked on U-boot.

Multiboot flashes kernels. If the flashed kernel already contains U-boot, then you obviously continue to have U-boot.

So when you say "I could flash other Maemo kernels (with multiboot) and u-boot was still available" it means either you told multiboot to flash a kernel that itself included U-boot, or (unlikely) multiboot is doing some behind-the-scenes magic to append the kernel to an existing U-boot if found. Very unlikely.