It was the employer that wrote the at-will terms into the agreement. If they don't like their own terms, Not My Problem. For me it depends on two things: how satisfied or annoyed I am at my current position, and how anxious the new position is to have me start. If I'm relatively happy with my managers and co-workers and it's just that the new position's offering me better pay or different work, I'm going to push for 2 weeks notice before I start the new position just out of courtesy. If my current employer's willing to write a certain amount of notice to me into the agreement (ie. they won't let me go without at least X weeks notice), I'm definitely going to insist on giving at least that much notice before leaving. OTOH if my current employer insists on being able to let people go at any time for any reason with no notice, I'm going to be less than insistent on giving them notice. If I'm annoyed with them, and especially if the new position wants me right away, I'm not going to lose any sleep about giving them exactly as much notice as they give employees being let go (that is, none at all). The only consideration for me will be making the departure clean on my side, all my personal stuff cleared out, company data on my workstation safely backed up where my manager knows to find it if they need it, sensitive information that the company doesn't need (eg. passwords to linked-directly-to-me accounts needed for work, SSH/SSL/x.509 private keys) wiped, etc. etc..
If an employer has a problem with that, I suggest they review the idea that I'll grant them exactly the consideration they grant employees. If they don't consider their terms acceptable, it's entirely within their power to change them. If they expect me to grant them consideration without granting anything in return, I refer them to the acronym "TANSTAAFL".