As a bit of a change from discussing the big 3...
Hitwise's statistics for Ask.co.uk in the 4 weeks up to May 26th 2007 show that Ask.co.uk has around a 5% share of the UK search engine market; just behind MSN (see: Heather Hopkins - Hitwise UK: Panama UK Launch - Updated Search Stats). This, coupled with the site's recent redesign and new functionality mean that it should afford it more than a passing consideration when optimising.
After a short time reviewing a variety of results today, it does appear that it values backlinks / site popularity less than it does use of keywords in title, Meta & # of uses in body copy with extra credit for recurrences of the search phrase in order. Obviously these are important on the other major engines too; but site's I would expect to appear in top positions barely rank at all.
Does anybody else have experiences of optimising for Ask?