Ya, so i had to troubleshoot a user who was not able to access a custom object.

The user was assigned a standard profile. I edited the profile, and found out that i cannot edit custom object permissions for a Standard profile.

This new change has come into effect from Spring '10 for increased security reasons...

Click here to check out the discussion page from where i came to know this.

Spring '10 release notes. Page 188 states as below


Spring '10 introduces enhanced security for custom object permissions. In Enterprise, Unlimited, and Developer Editions,
when you create a custom object, the “Read,” “Create,” “Edit,” “Delete,” “View All,” and “Modify All” permissions for that
object are disabled for any profiles in which “View All Data” or “Modify All Data” is disabled. This ensures that all users don't
immediately get full access to custom objects.
You can change these permissions in custom profiles, but not standard profiles. That is, users with standard profiles (except
System Administrator) can't access new custom objects—you must assign them custom profiles and edit the profiles.To enable
access to custom objects, do one of the following:
• For users with standard profiles:
1. Clone the profiles of the users whose object permissions you want to change.
2. Edit the custom profiles, enabling the permissions you want.
Tip: If enhanced profile management is enabled for your organization, you can edit multiple profiles from the
list view.
3. Edit the users' accounts, assigning the appropriate cloned custom profiles.