You want to upload a rigged mesh, but you get trouble ?

Here are some hints:

Not all vertices have been associated to weight groups:

In that case the importer should report back an apropriate message.
But i suspect that it stopped to do that and silently disables the upload button.

Not all weight groups have been defined:

The SL skeleton consists of 26 bones of which 21 are essential (as of my understanding).  The weights for each bone are stored in a vertex group (weight group). The list of essential bones is (bone names identical to associated weight group names):

mHead
mNeck
mCollarRight
mCollarLeft
mShoulderRight
mShoulderLeft
mElbowRight
mElbowLeft
mWristRight
mWristLeft
mChest
mTorso
mPelvis
mHipRight
mHipLeft
mKneeRight
mKneeLeft
mAnkleRight
mAnkleLeft
mFootRight
mFootLeft

The remaining bones “non essential” bones:

mToeRight (weight group seems to be ignored )
mToeLeft (weight group seems to be ignored )
mEyeRight
mEyeLeft

If your .dae file contains the 21 weight groups named identical to the essential bones in the list above, then the upload should always work. Recently the behaviour of the SL viewer changed from version to version. Hence many people give the advice to always include the weight groups for all essential bones. The groups can be empty! But the intended behaviour of the viewer is to accept dae files with any subset of the bones. For me the latest mesh development viewer works as expected, but the official mesh viewer wants to see the 21 essential bones.

The mesh is only partially UV-unwrapped:

In that case the importer should just import the mesh. But it was reported that it returns an error about “too few materials”. I myself have not seen this behaviour but i have never tried to upload a partially unwrapped mesh. If that is your issue, then maybe it is worth a bug report ? The importer should report why it fails.

The mesh can be uploaded, but is massively distorted:

Check if your mesh object has been rotated (and scaled) in object mode. If that is the case, then you should apply rotation.

For Blender 2.5: Object -> Apply -> Rotation
For Blender 2.4: Object -> Clear/Apply -> Apply Scale/Rotation to ObData

As soon as a physics mesh is added, upload won’t work any longer

There has been a change recently which forces the physics mesh to use at least one material from the LOD’s. This change has been reverted in the latest mesh development viewer as it was recognized as a silly idea. So your workaround is to use the latest mesh development viewer or provide the needed material for the physics shape and try again.