Cite articles "in preparation" or conference presentations in post-doc/junior positions grant application

As noted by @aeismail, there are many grant mechanisms that expressly prohibit listing anything that's not "in press" or published, so the first stop would be checking the rules for the grant application itself.

If it is allowed, it's understandable to a junior scientist to do this (and indeed, I did this on several job applications). A few general notes:

• Don't list a journal. No one cares that something is "in submission" to Nature or Science or Cell or what have you, and it erodes your credibility.
• Do be sparing in what you list. These should be papers you're actively working to prep for submission, are being circulated to co-authors, etc. Things where in another few months your CV might look markedly different. "I've written the title and maybe half the intro" doesn't count. Every idea and musing you've had doesn't count. There shouldn't be a wall of these, or way more in prep entries than actual published work.
• Do be prepared to talk about each and every one of them (this follows from the previous point). I have been asked about papers I had in prep in a fair amount of detail (they were interested in the topic).

In general, most grant applications do not permit you to list unpublished work for consideration. This is to prevent "gaming the system," whereby somebody could claim a whole bunch of work is "in preparation" when it really isn't. However, you need to check the specific instructions for each funding source to see what their rules are with respect to unfinished or unpublished work.