[Jason Abbadon]

If Aliens are coming here..they really suck.

Consider: the solar system is vast- a ten kilometer ship- the size of the Executor from Star Wars- would be the smallest black hair in a deep dark well.
That being the case, Aliens could get anyting they need about us- including detailed DNA breakdowns- from our transmissions.

A ship hanging out just beyond the asteroid belt would have a few hours of lag time in listening in on our chatter, but that's pretty minor.

All of our internet traffic would be available to them.

Then there's the plausibility of sendng organic life out exploring- out air, with all it's viruses, bacteria and pathogens that evoloved along with us would be extremely hostile to aliens- and close proximity to them would likely be as bad for us.

But machines...they culd be out there, doing their thing.

Greg Bear has two impossibly great epic books- Forge of God and Anvil of Stars that deal with this notion- that "Von Neuman" probes could listen for inteligent transmissions, investigate and self-replicate by useing raw materials in a solar system to make more probes, which would do the same and continue the bigger mission.

The second book, Anvil of Stars is simply, the very best book I have ever read.
Period.