Data consistency in the cloud is a totally different ball game - as perfectly illustrated by this newsletter issue from ZapThink. Essentially, you have to cope with some form of partition tolerance and stale data values in caches. This is usually summarized as the CAP issue (for more details, see the ZapThink post).
Luckily, the solution to the problem can be as simple as outlined in our solution to the CAP problem.
Life is good