I uninstalled XPSP2, uninstalled the Lonestar alpha, reinstalled XPSP2 and my TIP came back. Amusingly, I found this by Googling a discussion board from a few months ago where the person asking the question was breaking NDA and a Microsoft representative was very politely, but very insistently, posting "This isn't the appropriate forum for discussing this," while the problem was being discussed by others on the board.
Which raises the question: Do alpha / beta NDAs make sense anymore? Certainly there's a period when products / features must remain "stealthy" but with betas becoming very marketing-oriented and even "technology previews" such as the PDC bits becoming available, does commercial sotware really benefit from the NDA until product release?