I also would like to express my support for a on-the-fly-spellchecker. It would bring LyX in line with other similar editors, where such a feature has been standard for many year. A feature poll in our wiki has indicated an overwhelming support for it by many LyX users. Although the implementation of new features needs to be carefully weighted in terms of required work effort, consideration for finally implementing an on-the-fly-spellchecker in the 1.6. series should also take into account its neccessity and common place.
Many thanks to the brave team of LyX developers, who are always confronted with hard decisions to make! Cheers, Sam