Unable to change Apple ID on my iPhone I changed the e-mail address linked to my Apple ID. Now when I try to update apps or purchase them on my iPhone 4, the old address pops up asking me for the password. I cannot seem to get it to change on my phone. I can log into iTunes and purchase apps there on my computer without difficulty.

I heard some people say that your iTunes ID is different than your Apple one, But I cannot see (or find) a separate iTunes ID... when I click on my account for iTunes, it shows me my Apple ID. Any ideas?  Anyone know why it will not change on my phone but will in the computer?

I downloaded and installed iOS 5. I have already tried syncing my iPhone.
Your iTunes ID is the same as your Apple ID. I've recently switched email addresses as well, so I can confirm that you can easily change the email address.
Assuming that you've already successfully changed your Apple ID, follow the steps that Hand-E-Food mentions:

Go to Settings

Select Store (near the bottom of the section starting with General)

Click on your (old) Apple ID

Click on Sign Out

Click on Sign In

Click on Use Existing Apple ID

Sign in with your new Apple ID email address and password
That's all there is to it!