I don't think it signals trouble for Rob, one way or the other. Phillip might try to oust him near the end, but I doubt it once he sees the resentment brewing against Rob.
Earlier in the season (like Episode Two, I believe), Rob talked about carrying Natalie on his shoulders to the end. Later, he talked about taking Natalie/Ashley, and now, Phillip/Natalie, of course. If Rob's Final Three comes to fruition, Natalie will become a millionaire. Most of Zapatera along with some wronged members of Ometepe (such as Matt), will be jealous/angry of Rob and refuse to give him the check. Basically, Natalie is the new Amber.
Natalie Votes: Ashley/Matt/Julie/Ralph/Mike/Steve/Andrea?
Rob Votes: David/Grant
Phillip Votes: none
Rob will have a fair shot at winning if the jury doesn't start until Final 10. If the two losers on Redemption Island are sent home, rather than to Ponderosa, then he stands a much better shot since two of those people are very unlikely to sign their name to his check.
Rob losing at all would make him the most wronged person in the history of the show. No question.