I have a Clio Dynamique TomTom including the uprated Bluetooth stereo and I'm pairing it up with an iPhone 4S via bluetooth.
Before iOS 5.1, I could skip foward but expected a delay of about 5 seconds, though if I left the track playing for a while it'd sometimes skip forwward instantly but the delay meant that it was practically impossible to skip backwards at any point. Also, if the sat nav cut in with the voiceover or alerted me about speed cameras, it'd be a good 5+ seconds before the audio started to resume. I also had no access to track information, so it'd just rad 'Audo track' or words to that effect.
Prior to iOS 5.1, skipping forward is pretty much the same, though form time to time if I'm lucky I can skip backwards if it's in a good mood, but again it's a challenge to say the least. Now the two things I have noticed are that track information displays now at the bottom of the TomTom screen, or at least I get the track title, but it's somewhat delayed. Unless I try poking around it'll stick with a track I played about 3 or 4 tracks ago. I'd say one welcome update since iOS 5.1 is that pausing/resuming is instant every time I've tried it. I've since disabled voiceover with the TomTom because it does my head in, but the speed camera alerts cut in as expected and the audo resumes pretty much straight away afterwards.
I was on the Apple Developer connection site when iOS 5 was in beta release and ever since I upgraded from iOS 4 I've had this problem. I've also had an open bug report (#10058939) with Apple since 1st September 2011, with attached logs etc. I've had no replies to this report, however I will continue to update it with my findings as I go along. I had previously had a report open whereby they did respond about the issue, however I was unable to get the logs to them in time before the ticket was closed, though they did mention to me something along the lines of them working with Renault in an attempt to resolve the problem.
I tried my phone in a Skoda Yeti a few months back with iOS 5 and it worked perfectly so I assume it's some kind of incompatability with the Renault stereo systems.