I think we need to be a little careful about writing off the security warnings for RIM, as per the post above. I am sure you will have no problems, but there is a theme of instability in the country, which sensible travellers will consider.
Without reciting government advisories, there was the death of four French tourists in Aleg in Dec 07. 2008 saw the shooting up of several nightclubs and the Israeli embassy in Nouakchott, along with a shootout in NKT between police and an escaped suspect of Al-Qaeda in the Magreb. 2009 has seen a bloodless coup, along with shooting of an American teacher and an attack on the French embassy in NKT. There has also been longstanding instability on the border zone with Mali, albeit this is usually some distance from the tarmac road via Nema.
Whilst there are rumours that the Dakar rally was relocated in 2008 due to commercial pressure, there was clearly instability in the region which no doubt contributed to ASO's decision to relocate.
Having been to RIM several times in the last few years, little of this is noticeable on the streets on Nouadhibou, Nouakchott or Atar, but the fact remains that these risks exist. If you walk past the Israeli embassy (or linger via the Senegalese embassy on the other side of the road) you will be quickly ushered away, which is a remnant of the previous attack on the embassy.
The basic premise being that Mauritania is generally a peaceful and rewarding country, but recent events in the last three years have created some issues. The vast majority of travellers will not be affected by these, but I think it would be remiss of this forum not to consider them and make people aware that parts of the country are potentially unstable.