._file files aren't actually temporary files but a file that holds the resource fork data for Mac OS X. Mac OS X will create this for files that are being stored on a non-HFS system in order to preserve the metadata.
However you can run the following command to disable the creation by TextMate of a resource fork:
defaults write com.macromates.textmate OakDocumentDisableFSMetaData 1
If you have the latest version (build 1589 - Nov. 23rd, 2009) it also allows you to enter an array of file systems instead if you still want the metadata on other drives (say yes to your internal, and no to any network or non-HFS drive):
defaults write com.macromates.textmate OakDocumentCustomFSMetaData '( afpfs, nfs, msdos, hfs )'