Hi Gaute,
I've heard vague references to F5 considering a tie-in with an antivirus company so you could potentially do virus scanning locally on LTM. You might check with your F5 account rep to see if there is anything more concrete about this now.
Else, you could try to send a full payload or HTTP parameter to a sideband AV server for validation, but you'd be limited to 4Mb--so I don't think there is a practical way to do this with LTM right now. See this post for details:
Virus scan attachments
http://devcentral.f5.com/Default.aspx?tabid=53&forumid=31&tpage=1&view=topic&postid=3081430847
Aaron