Hi friends,
Its almost been three years I have been working on db2 LUW. Now I have an opportuninty to work on AS/400.

1. Is the administration of db2 on LUW and AS/400 same or is completely different?
2. Only OS level commands will defer or db2's behaviour(syntax-wise or execution-wise) will also be changed?

How do i start, i am given only a weeks time to explore the differences; will it be possible for me to understand and take the responsibilities given a weeks time?

Administration of db2 on iSeries is completely different, it's easier to say about differences than similarities. Db2 on iSeires is integrated with operating system i mean if you install os you have db2. so, no tablespaces, containers, bufferpools etc. Engine i think is almost the same(sql, optimizer, governon, access plans).
But again even in SQL there is some differences.
From administrator point of view it's more simple to administer db2 on iseries. There are only operating system command and tools to do it. If you want same more information let me know i will send you some links...

Thanks iksior...Please also send me the links.

you should find everything here:
IBM DB2 for i