mmmm, are you sure about that external SD card bootmenu item ?
When trying that I'm getting in U-Boot :
And I'm being dropped in the U-Boot console.

The 3rd partition (the FAT one) on the SD card does not contain a boot.scr file, only a kernel config text file, a kernel memory map text file, the vmlinuz version of the kernel and the uImage version of the kernel. No boot.scr file.

What are we doing wrong ? Does this work on your N900 ?

PS. In the mean time received I my Samsung 8GB Class 6 SD card and I used that one as the target for the image, writing was fine. As opposed to the class 10 SD card, which seems like f*cked up. rewriting the image took forever and the card reader activity light was very strange behaving. I'll try to recover that one later.