Using the flash.text.Font.enumerateFonts() routine one can see the following fonts available (at the moment... back at SDK beta 2) in the PlayBook simulator (skipping over some Japanese, Thai, and Indian fonts, and grouping related ones with slashes, to keep the list shorter):

1. Andale Mono
2. Arial
3. BBAlpha Sans (plus Condensed/Mono)
4. BBAlpha Serif
5. BBArabic Keypad
6. BBGlobal Sans
7. BBGlobal Serif
8. Bitstream Vera Sans (plus Mono)
9. Bitstream Vera Serif
10. Comic Sans MS
11. Courier New
12. DejaVu Sans (plus Mono)
13. DejaVu Serif
14. Droid Sans Mono
15. Georgia
16. Impact
17. Liberation Mono/Sans/Serif
18. Tahoma
19. Times New Roman
20. Trebuchet MS
21. Verdana
22. Webdings
23. Wingdings (plus 2 and 3)

It seems they're also taking requests for other fonts to be available in the final release.

[Edit: a handful of new fonts was added in the 0.9.4 release, and most importantly the default font changed from "BB Alpha Sans" to "Myriad Pro".]