Here are two examples:
Define the actual text using the textPs command as normal, then include the textPath where the text should "flow:"
http://s7d2.scene7.com/is/image/TechSupport/?&textPs=\fs50\ul1\stextflow1{GGGGGGGGGGGGGGGGGGGGGGGGGGGGGG}&textpath=M264,210l-8,240,421,-51,227,33,170,72,z
http://s7d2.scene7.com/is/image/TechSupport/?layer=0&anchor=0,0&size=2000,2240&opac=0,100&layer=COMP&layer=0&color=0xffffffff&opac=0,100&layer=2&layer=3&textpath=M-500,0%20C-500,500%20500,500%20500,0&textps={\fs100%20$A${\fs200%20$B$}$C$}&$C=CCCCCC&$B=BBBBBB&$A=AAAAAA&pos=1000,0&layer=COMP
For more information, see the IS documentation:
http://microsite.omniture.com/t2/help/en_US/s7/is_ir_api/index.html#textPath