Cloud apps are starting to be everywhere these days.
Mozilla is realeasing Bespin an online code editor based only on HTML 5 technologies (mainly Javascript).
You can play with it here or grab the code to see how it is done.

I think that this kind of software is very interesting.  I played with it a little bit and I think that it is well done and includes some very nice features (it is pretty fast, scales well etc...).

But still, from a security point of view, I remain pessimistic about the privacy issue. The files are stored and modified in the cloud.

A very interesting post explains the security issues of Cloud-based apps.
It is the new emerging internet application architecture. Microsoft is one the main actor promoting cloud apps, with its new  Office Live online tools. Another good example of cloud app is the google apps.
The post explains the cloud computing security issues, discussing what are the security aspects applied to web application that cannot be applied to cloud apps.

It is interesting to see that cloud apps require a deep modification/adaptation of security tools and methods.
Another important problem is I think the privacy.  We will end up with having a 'big big brother', more than any other one before. Cloud apps allow the files (data) to be stored in server database, and the applications; like word or excel etc. to be launched from the internet.  i am not sure how much we are able to trust the servers storing all these information and able to monitor the user access to these apps.

I think that, privacy is a very important problem to tackle when discussing about cloud apps.