Session expired or invalid immediately after successful OAuth login

Hey everyone,

I'm working on an application, and I got the OAuth2 workflow working right away, but I've been struggling with making requests afterward.

I get the access_token and instance_url back, and when I make a call like this:

curl -v https://__instance_id__ (na7).salesforce.com/services/data/v20.0/ -H "Authorization: OAuth access_token_from_earlier"

the server responds with a 401 with the following body:

[{"message":"Session expired or invalid","errorCode":"INVALID_SESSION_ID"}]

I've scoured the forum for solutions already, and made sure that I have API access turned on, signed up for the REST API through the developer preview form, but am still having errors all over the place.

Any help would be greatly appreciated.

Thanks!