I`ve an EeePC netbook. What filesystem should I choose for Ubuntu installation? I also want to make a partion accessible for ubuntu and win. What should I choose?
To acheive what you seek would involve a compromise in one way or the other, as for whether the compromise is acceptable probably depends what you intend to store on the shared partition.
There is not to my knowledge any filesystem that supports all the features you have come to expect on filesystems of each OS. The primary reason for that is each filesystem was built with a specific platform in mind (NTFS/FAT32 for windows, EXT3/4 for linux).
That said there are ways to get some level of shared acess and we come back to what you are storing. In general your best options are probably FAT32 or maybe NTFS, Linux support for these are more mature and in Ubuntu by default unlike windows support for EXT3/4. If like when I was dual booting the main files shared on that partition are such as media files or other non sensitive data, etc the loss of permissions are probably less of a big issue for you. That may also be similarly true if you are the sole person with direct access to the machine in question.
Another concideration may however be which system you use the majority of the time, there may be something to be said about choosing a filesystem format that best supports the OS you use most often. Though I don't know enough to recommend the EXT3/4 support options that can apparently be added to windows, if they are not sufficiently mature data loss could be a risk. Do your research before trusting any software with that level of filesystem access.
Of course another question is do you need a fully shared file system, how much data are we talking and could such as a USB flash drive be used to hold this data? That would eliminate the permissions issue down to looking after the physical device.
That should give you a few options to consider, I advise having a think about what you need doing your research of the limitations and benefits of each option and select the one which best suits your situation.