For example can I ask someone to thumb up my site if I thumb up theirs? Or Digg my site if I Digg theirs? Will this kind of thing get you banned?
The way which you are asking if digg moderator came to knbow then they will banned your account. But you ahve hae reliable friend and those are interested to help each other then it's ok
Best thing is to try it. If people respond positively, then you can proceed else drop this idea. But, it is very hard to do like reciprocal linking in digg and social bookmarking since people like to bookmark those sites which they like.
The only thing that could happen is a ban on the site, either if people report you for this techniq, or if the site recognizes it. Others here have said that search engines could recognize this technique, which is absolutely false.