There have been changes to the metadata API for
MobileSettings in Spring '13 that are causing the problem for you (see below for more). The issue is that the sandbox is on version 27.0 (Spring '13) and your IDE is still on 26.0 (Winter '13). The version 27.0 IDE will likely not be released until after all of the production instances are upgraded. When it is you can upgrade your IDE and then run your project through the IDE's upgrade wizard.
Check out the Spring '13 release notes, page 112
In Spring ‘13, several Metadata API types have been updated or added
to improve support for organization settings. For the declarative
file-based API, organization settings types are accessed via the
Settings name and setting component member.
For example, the following excerpt from the package manifest file
accesses SecuritySettings inforrmation:
SecuritySettings and MobileSettings are no longer available in API
versions 25.0 and 26.0.
The following excerpt uses API version 26.0 syntax for
SecuritySettings and will fail if used in a retrieve or deploy:
You must remove the
MobileSettings from your
package.xml file or use the new API version
There is a question on how to override the API version in the ide or alternatively you could look at the force.com ant migration tool. If you do anything where you manually edit the package.xml in Eclipse make sure that you uncheck whatever components you need to in the GUI to make sure that the problematic XML doesn't get generated again.
Interestingly, there is an idea to open source the IDE and SF product management mentions that SF is hard at work on the tooling api (btw, awesome!!!), also released in Spring '13, and that it would allow for an open source implementation of an Eclipse plugin.