Jason Adams has posted a Python script that does exactly that, posting your tweets as plurks and your plurks as tweets. I have passed this info on in Twitter and Plurk but have had a couple of less techie types come back and ask just how to use it; so here is a description. Please note that this assumes you are using Windows and things may be rather different if you use Mac OS X or Linux.
Go to python.org and download the Windows installer from the link on the left of the page.
Right-click the python-2.5.2.msi file once it has downloaded, select Install and got through the steps to install; choosing the defaults is fine.
Once you have Python installed, go to Jason’s Plurk your tweets page and download the plurk.py script file. Remember the folder you saved it to. For the sake of this example we’ll assume that you saved it to C:\Utils.
Open a command window and change to the folder you saved the plurk.py file to (C:\Utils).
c: cd \utils
Now you are ready to run the script, which takes your Twitter and Plurk credentials as parameters. Assuming that your Twitter user name is t_usr, your Twitter password is t_pwd, your Plurk user name is p_usr and your Plurk password is p_pwd; run the command as follows.
plurk.py t_usr t_pwd p_usr p_pwd
The script will now run and takes a while the first time, but subsequent runs will be quicker. The first time you run it the last ten or so plurks and tweets will be synchronised and the state of synchronisation will be stored in the plurkdb.dat file in the same folder as plurk.py. In future it will be used to ensure that only new tweets and plurks are synchronised so it is important that you move it too if you ever decide to move plurk.py to another folder.
I hope that this answers your questions; otherwise leave a comment and I’ll try and provide further assistance.