Even though undrafted free agents aren't allowed to sign with teams yet, deals are already being made at a rapid pace. Think of Tuesday as high school signing day for recruits. Right now coaches from across the league are making their pitches to undrafted free agents that they are targeting, and handshake deals are essentially being made. Nothing is official until Tuesday, but lots of agreements are happening as we speak.
Already the Lions have agreed to a deal with Alabama tight end Preston Dial, and you can now add Northwestern linebacker Quentin Davie to the list. He tweeted this a half-hour ago:
I'm Officially a Detroit Lion!!!!!less than a minute ago via Echofon 
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Quentin Davie
QDavie41
Davie played in 13 games each of the last three seasons and 10 games in 2007. He had 71 tackles in 2010, 90 in 2009, 57 in 2008 and 22 in 2007. During his career he had a total of 10 sacks, 14 pass defenses, four interceptions, five forced fumbles and three fumble recoveries.
Considering the Lions are not exactly overflowing with a ton of talent at outside linebacker right now, there will certainly be an opportunity for Davie to make the team if he has a good training camp and preseason.