I've spent the last few days being blue-rinsed by IBM on p6 & AIX 6. Very nice... As well as the v. desirable new hardware, AIX 6 (nee 5.4 :-) has got some damn clever tricks too.
IBM's LPARs let you carve your machine up as you see fit (starting from POWER4). POWER5 brought in the micro partitions (see Cesar's comment), which let's you finesse things considerably.
AIX 6 brings in Workload Partitions (WPARs), which let you effectively slice your O/S instance even further (I think this is Sun's containers style operation?). Because it's AIX 6 doing this, the function is available on POWER 4, 5 and 6 (but not older kit) - you *don't* have to have p6.
Maybe a good way to tell how this plays is to see how the competitors (HP, Sun) try to play down IBM's announcements. Right now, I can't see any real compelling reasons not to play the IBM game, but then I may be a bit biased (no, I don't work *for* IBM, but I've spent a whole lot of time working *with* IBM - anybody remember 6150 / RTPC?).