Sounds like a permissions problem to me.
You can run a permissions repair from Disk Utility or :
You can just open up Finder press Shift+Command+G and it should bring up the Go to Folder dialog. Enter this in: "~/.Trash" without the quotes and then click Go.
Now go to File->Get Info and then scroll down to the bottom when it says Sharing and Permissions. Look for your name. If it says "Read Only", then click on the little lock at the bottom, enter your password and then change it to "Read and Write". I think that will fix it.