Are you a local administrator on the sharepoint box? If not what is your permissions level? Sometimes this behavior can be caused by permissions level. There are a number  of other reasons why you could be experiencing this kind of behavior as well. Something such as a free form text fields, attachments, or calculated fields sometimes cause this. I am sure someone might be able to pinpoint your problem better than I can. Maybe a screen shot or a list  of the different data types that show up in the all items view  that you are trying to edit. Another thing you could try is export to excel spread sheet update a few items and then use the sync command button the list toolbar to sync the data back to sharepoint with your local changes. If this works  for you fixes your error you could do them all that way. This way you don't time out on the webpart page making changes and lose a bunch of data.

Another question  in addition to your permission level: When you open  in
datasheet view, do you get any messages that tell you that some
required fields are not shown in that view? That will also put the view
into read  only mode.
The best view  to use as a datasheet  view is AllItems.aspx as long as
all custom columns have been added to the view.

I’ve been having the same problem. I DON’T think that I have all required fields in the view, but I don’t get any messages. I will try that !

Is "Content Approval Required" set to "Yes" for this list?
Unfortunately, the only way I found to get around this was to set it
to "No" while I made  the necessary changes in the Datasheet View.

Hope this helps. Let me know if you found a different way to solve
this issue.

I have a similar problem. I created a "form library" through InfoPath. After data is loaded via InfoPath to the table, I export the datasheet  table to Excel, but the table comes out as read  onlo. Not only is it read only in Excel, BUT also in the Datasheet view  (all items). While I don't want to change data or do mass uploads I would like the fucntionality of editing a few fields through Excel to be available