GlobalFellow,
If you will be applying in EB3, then you will most probably have no choice but to file an I-140, and wait until your priority date is current before filing your I-485.
If you qualify for EB2 or even better, EB1, then read my note below.
An I-140 is filed by your company, not YOU so simply filing an I-140 does not show clear immigrant intent, since it’s the company filing in your name. There was a USCIS memo that was issued that clarified that simply filing an I-140 does NOT show immigrant intent. The company "could" file it without your consent, so therefore it’s not your fault that "they" want you to immigrate, eventually. It doesn’t necessarily mean you want to immigrate. By filing an I-485 you are clearly expressing YOUR choice to immigrate, not anyone else's, YOU are the one that consciously made the decision to immigrate by filing an I-485, and that is clear immigrant intent.
By filing an I-485, you show clear immigrant intent. For this reason, once you've applied for an I-485 you are no longer eligible for a new TN, since to qualify for a new TN, you cannot have dual intent. Dual intent is defined by having shown the intentions of wanting to switch to a permanent status (i.e. Green Card), while still in a temporary status (i.e. TN, H1b, L1, F1, B1, etc). Only H1, and L1 visas explicitly allow for dual intent, and so people can extend their H1 and L1 visas while their I-485 is pending. TN, F1, and B1 statuses do not afford you the right you to extend your status while your I-485 is pending. People in statuses other than H1 and L1 that have a pending I-485 must use an EAD to work once their status is expired, or gets voided by leaving the country, since statuses like TN do not allow you to renew once you’ve applied for an I-485.
If you file the I-485 and the I-140 concurrently, and the I-140 is DENIED for any reason, then once your TN has expired, you are out of status, and you are no longer eligible for a new TN because you've shown immigrant intent. You can call it game over, and pack up and go back to Canada. Although a good lawyer should help to ensure that an I-140 does not get denied, lost mail, disorganized USCIS practices, and unskilled adjudication officers are just a few reasons that are out of your control as to why your I-140 may inadvertently get denied.
On the other hand, if all you filed is your I-140, and it gets denied, you can still apply for a new TN, and re-file the I-140. Life goes on, and you learn from the mistakes you made on the I-140, and you try again.
Also, if you filed concurrently and the I-140 gets an RFE (Request for Evidence), and then I-485 can be put on "hold". With the I-485 on hold, this may cause a delay in getting the EAD, and AP.. If the TN expires during this time, you would not be able to work, since you would not have an EAD yet. Examples of this are the ability to pay RFE's for smaller companies, or education issues for EB2 cases. These are common RFE’s that are predicable, yet unavoidable in some cases, even with a great lawyer. The delay due to this RFE on the other hand is unpredictable, and this is where the issues arise.
Next, you could get your not-yet-approved I-140 inadvertently transferred to a district office, because the service center wanted to give you an interview for your concurrently filed I-485, not realizing that the you have a pending I-140. Although a district office does have the capability to adjudicate an I-140, in general, they will claim it is the responsibility of the service center, and will not even touch the case. If you were on a TN in this scenario, you would be caught in the loop of trying to get your I-140 back to the service center, all the while your EAD may not get approved before your TN expires because nobody at USCIS wants to admit to having your file. Again, bad luck due to USCIS error, and it’s out of your control. The risk is too high to take the chance.
Note for EB2 and EB1 applicants with current priority dates:
Even if your priority date is current, then if the current processing times at your service center are more than 9 months to process an I-140, then you have no choice, you can't file concurrently when adjusting from a TN. (math: 9 months to process I-140 + 3 months to process EAD = 12 months which is the maximum time allowable on a single TN) In this scenario, your TN will expire before you get an EAD in your hand, since your EAD application would be put on hold until your I-140 is approved, and since you filed an I-485, you're not eligible for another TN. A person in H1b would not run into this problem.
I am not a lawyer. But, my recommendation from what I've seen and read, is that someone who is in TN status and plans to file for AOS, should NOT file concurrently due to the fact that a TN does not afford you the right to dual intent.
Read more here, on the second post on this thread:
http://www.immigrationportal.com/showthread.php?t=152852