Is there a way to touch() a file in Amazon S3?

You can achieve the same through a copy object request, specifying the CopySource to be same as the target key.

In essence, this will issue a PUT Object - COPY request to S3 with the corresponding source and target bucket/key.

In response to @Daniel Golden's comment on @tkotisis answer. It looks like at least the AWS CLI tools do not let you copy an item on to itself. You can however 'force' a copy by updating the metadata.

$ aws s3 cp --metadata '{"touched":"now"}' s3://path/to/object s3://path/to/object

This recreates the object (downloads to the caller and reuploads it) replacing its content, owner and metadata. This will also trigger any attached Lambda events.