What Fortune 500 companies are active in base of the pyramid markets?
The following is an incomplete list, however it is a list that caught the eye of Harvard professors who study and teach about base of the pyramid issues. (Link)
1. Briston Meyers Squibb
2. British Telecom
3. Cisco Systems
4. Citigroup
5. Coca-Cola
6. Deutch Bank
7. Danone
8. General Electric
9. General Motors
10. Glaxo Smith Kline
11. Google
12. Hewlett Packard
13. Hindustan Lever
14. Kellogg
15. Mattel
16. Merck
17. Microsoft
18. Motorola
19. Nestle
20. Nike
21. Nokia
22. P & G
23. SC Johnson
24. Siemens
25. Starbucks
26. Tata
27. Unilever
28. Vodacom
29. Walmart
30. Toyota
31. Hyundai

Here is an interesting side bar on innovation models–from the perspective of GE
http://www.businessweek.com/magazine/content/07_11/b4025417.htm
By the way, GE focuses its efforts on the following (at least in Brazil):
1. energy
2. health care
3. transportation/infrastructure
Innovation for GE looks at need & the customer.
They look at the major bottle necks in development–and work with the government as a customer.