Bootmenu is a script that the /sbin/preinit script executes during boot. Basically, it checks if keyboard is open and if the script exists at /bootmenu.sh, and runs it if both are true. The bootmenu package is a specific bootmenu.sh script which includes the necessary 'goodies' to make a menu for choosing what to boot. Multiboot was a different package that came later but as I undertstand it, Multiboot is derived from Bootmenu and uses similar methods.

U-Boot is completely different - UBoot is a universal bootloader, very widespread in use - wheras Bootmenu and Multiboot are Maemo-only as far as I know.

U-Boot has more technical flexibility but it's less newb-friendly (well, more like Multiboot/bootmenu were documented better for the newbs first) than multiboot and bootmenu. Also currently Multiboot and Bootmenu have to reflash the kernel every time you change OSs, which is flash chip wearing in the long term. UBoot can actually load kernels from ANYWHERE without flashing. (Realistically, your flash chip will survive hundreds of thousands of kernel reflashes, so it's not a major concern.)

But yeah, there's that. Then there's kexek, which is one kernel loading another. It's available in the repository too but there's a lot of work between one-kernel-loads-second-kernel and end-user multi-booting. Pali was working on kexek earlier from withing his recovery console item for bootmenu (speaking of, Pali, could it be that kexek wouldn't work from within your recovery shell because bootmenu runs stuff chrooted [based on what I read]? Or did I already ask this and get your answer in that thread? I may have just forgotten due to excess stupidity).

And yes you can triple boot Maemo, MeeGo, and Android. No I'm not saying how because it has been asked before and I've typed out long-worded explanations of how before. Basically, learn how both uboot and multiboot work, which will tell you that the two are c semi-compatible, and then use your understanding to combine the two correctly.