1- There are several things you must consider. Some servers simply can't handle ReST. So be sure you can use it.
2- User name and password must be over SSL and not through HTTP. So take care of that.
3- When you use SSL, certificates are a problem as you will be having self-signed certificates in your app. This will throw exceptions. So either disable certificate checking and risk your server or use create a fake cetificate maager in your device and call it everytime you connect(this is also risky) or create a new certificate as given in this link http://blog.antoine.li/index.php/2010/10/android-trusting-ssl-certificates/
4- Use these links to check that your certificates are in order and consume the web-service.
http://vipsaran.webs.com/TestTrusted.java
http://blog.synyx.de/2010/06/android-and-self-signed-ssl-certificates/
Apache HttpClient on Android producing CertPathValidatorException (IssuerName != SubjectName)
http://groups.google.com/group/android-developers/browse_thread/thread/62d856cdcfa9f16e/dd59b1998d23a660?lnk=gst&q=SSL+fake+cert#dd59b1998d23a660
Any problem, post further questions. It can be solved since you are using ReST.
[The links may have codes that aren't working. So they are only for the sake of reference]