Although by definition this is almost certainly going to imply long, slow cooking, I think it's fashionable at the moment to emphasize the low temperature aspect; compare with slow-cooked dishes like 'agneau de sept heures'.

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Note added at 37 minutes (2011-05-19 11:12:52 GMT)
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By 'low-temperature' here, we usually mean something between 50 and 80 °C

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Note added at 1 heure (2011-05-19 12:24:31 GMT)
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To answer the objections by Antonio, "what makes me think I know" about the cooking temperature is that I am currently very involved with modern restaurant cooking techniques here in France, and my partner is a top-notch professional chef.

The reason I discounted 'simmering' when I answered is that this refers to a particular style of cooking, specifically immersed in water (or other cooking liquid, of course!) — and this is a highly unlikely method of cooking to use for a rack of lamb, which would much more usually be roasted; it would be a crying shame to 'waste' it in a stew!

There are different ways of cooking it, of course; one current method is to put things into heat-sealed vacuum packs and cook them without added liquid, thus preserving their natural juices and succulence. Although this might not be the case here, it would certainly be a perfectly viable way of cooking a rack of lamb.

Regarding the temperature, from my experience of professional restaurant cooking and training, I simply know that 'basse temperature' (in this specific culinary context) specifically refers to a range of temperatures that are well below boiling point — i.e. as I've suggested above, something like the range 50 to 80 °C.

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Note added at 10 heures (2011-05-19 21:13:25 GMT)
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Naturally, the use of low temperatures does rather imply slow cooking; however, it is perfectly possible to perform slow cooking at higher temperatures too, hence why the two terms shouldn't be regarded, technically, as synonyms.

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Note added at 1 jour47 minutes (2011-05-20 11:22:33 GMT) Post-grading
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Thanks for the feedback, Moira!

As you say, your client seems to have confirmed that we are talking about the modern attitude to cooking, rather than the traditional idea of 'mijoter' etc. (delicious though all that is too!)