Yes, YuYu, academics can be an important part of your network even if they only have indirect contact to industry.  Remember that sometimes it's the indirect contacts that will be most helpful.  A professor may have a colleague or a former student (or a neightbor!) who might be helpful to you.  (That's partially why LinkedIn can be so useful - it lets you see who knows who. I do agree that you have to be sensitive about when you talk about opportunities.  I wouldn't start with 'Do you know of any jobs?'.  Instead, ask about general trends in the industry: "Where are your students finding work?", "What opportunities do you see opening up in the next several years?",  "What kinds of skills will graduates need to have in the future?".  If the speaker seems receptive, a personal note or email to follow up on specific opportunities might be in order.

You'll find that the quality and helpfulness of the sales reps varies considerably, Yuyu.  Some reps (especially if it's a busy conference) won't want to stop and talk with you.  Try not to interfere with their main job (sales) and approach them only when things are slow.  Ask them general questions about the company, how fast it is growing, what they like about working there.  Do they hire scientists into pre-sales (e.g. consulting) roles, or into post-sales support, or only into research?  Also ask whether there is anyone from the research side attending the conference and whether they will be doing booth duty and when.  It's certainly true that at big companies R&D and Sales will be totally separate entities (a mistake, I believe) but the sales people may have useful information for you. At some rapidly growing companies, employees get a bonus for bringing in new hires.  You won't have any trouble getting these people to talk to you! It's important to remember that not every networking contact is going to be helpful in getting you a job.  But each one of them will have some interesting information which could advance your search.  Look at networking as a research project: see what valuable information you can get.  But please be mindful of your contact's time.  And you'll find that in general most people like to talk about themselves, so frame questions like "How do you like working at Frammis Corp?", rather than "What's it like to work at Frammis Corp?".  Ask them how they got their job and what their background is.  Please be genuinely interested and don't be manipulative, and you should get good advice. Good luck!

It depends on what you are asking them to do.  If all you are asking is for someone to hand carry your resume to the person who is hiring, I've found that most people are willing to do that after only a casual acquaintance.  It never hurts to ask "Would you be comfortable delivering my resume to the head of Marketing?"  Of course, that's not a recommendation, but it's better than sending your resume into an email inbox.  If you are looking for an advocate within the target company, you'll have to establish more of a relationship.  Ask for their email address, and maybe try asking one or two of the questions in the pdf attached to this post.  Remember that most people love to talk about themselves, so ask about their backgrounds and how they got hired.  If they seem genuinely interested, then continue to build on anything you have in common professionally.  Be courteous and respectful of their time and be sensitive if it seems like they are putting you off.  Unless you have known someone for a while or worked with them, it will be hard for them to be an advocate. 

Some companies (especially in high tech) are paying employees a bonus for recommending people if they are hired.  You should have no problem finding someone in the organization to forward your resume at these places!