I followed the instructions on this side to update the boot loader of the Sheevaplug.

You will need a USB stick formatted with FAT16 not FAT32. Taking my 16 GB USB Stick didn't work for that reason ;-) So search for one smaller than 4 GB.

After successfull update and reboot of the Sheevaplug it displayed now the new version during booting. Also I realised that manually reset the device revealed the whole boot output:

  __  __                      _ _
 |  \/  | __ _ _ ____   _____| | |
 | |\/| |/ _` | '__\ \ / / _ \ | |
 | |  | | (_| | |   \ V /  __/ | |
 |_|  |_|\__,_|_|    \_/ \___|_|_|
 _   _     ____              _
| | | |   | __ )  ___   ___ | |_ 
| | | |___|  _ \ / _ \ / _ \| __| 
| |_| |___| |_) | (_) | (_) | |_ 
 \___/    |____/ \___/ \___/ \__| 
 ** MARVELL BOARD: SHEEVA PLUG LE 

U-Boot 1.1.4 (Dec 27 2009 - 22:03:21) Marvell version: 3.4.27

U-Boot code: 00600000 -> 0067FFF0  BSS: -> 006CFEE0
Soc: 88F6281 A1 (DDR2)
CPU running @ 1200Mhz L2 running @ 400Mhz
SysClock = 400Mhz , TClock = 200Mhz 
DRAM CAS Latency = 5 tRP = 5 tRAS = 18 tRCD=6
DRAM CS[0] base 0x00000000   size 256MB 
DRAM CS[1] base 0x10000000   size 256MB 
DRAM Total size 512MB  16bit width
Addresses 8M - 0M are saved for the U-Boot usage.
Mem malloc Initialization (8M - 7M): Done
NAND:512 MB
Flash:  0 kB
CPU : Marvell Feroceon (Rev 1)
Streaming disabled 
Write allocate disabled

USB 0: host mode
PEX 0: interface detected no Link.
Net:   egiga0 [PRIME]
Hit any key to stop autoboot:  0 
Marvell>>

Ok, now lets go ahead with Debian installation... :-)