How do you write the Italian equivalent of Bon Voyage? Or is there a
common expression in Italian that conveys the same good wishes?

Howdy, McCook!

The exact equivalent of "bon voyage" in Italian is "buon viaggio." 

"Have a good trip!
 Buon viaggio!"

Tiscali: Celebrations
http://www.tiscali.co.uk/reference/phrases/italian/data/celebrations.html

"Buon viaggio! have a good trip!"

Amazon: The Rough Guide to Italian (A Dictionary Phrasebook)  
http://www.amazon.co.uk/exec/obidos/ASIN/185828578X/202-1084638-8326209 

"buon divertimento: enjoy yourself, have a good time. 
 buon viaggio:  bon voyage.  
 buone vacanze: have a good vacation."

Really Find Education: Colloquial Italian
http://www.really-fine.com/Italian_colloquial.html 

Google search strategy:

Google Web Search: "bon voyage" + "buon viaggio"
://www.google.com/search?hl=en&ie=UTF-8&q=%22bon+voyage%22+%22buon+viaggio

Google Web Search: "buon viaggio" + "have a * trip"
://www.google.com/search?hl=en&ie=UTF-8&q=%22buon+viaggio%22+%22have+a+*+trip%22

Best regards,
pinkfreud

Five stars, as always with Pinkfreud