Hello Vasilis,
first of all, we have to note that encapsulation of PVST+ is totally different then standard IEEE 802.1D STP.
You can find very useful information about STP encapsulation and those of other protocols like CDP, DTP, VTP and so on in a document present in CCO named "best practices for IOS based switches"
see
http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a00801b49a4.shtml
IEEE 802.1D encapsulation has the LLC header with SSAP= DSAP = 42 with a well known L2 MAC multicast destination of
01-80-c2-00-00-00
Cisco PVST+ uses LLC SNAP encapsulation with LLC= 0x AAAA03 + 3bytes Cisco OUI= 0x00000C+ 0x010b as HDLC 2 byte protocol type with a multicast MAC destination of
01-00-0c-cc-cc-cd
To be noted on an access port a Cisco switch should use the IEEE version of the BPDU and the proprietary version is used over trunk ports
Hope to help
Giuseppe
Edit:
I have attached the pdf version of the document
