According to Wikipedia, NSLs used to prohibit you from disclosing their existence from anyone, including your lawyer. This was changed in 2006:
"On March 9, 2006 the USA PATRIOT Improvement and Reauthorization Act was signed into law... Other amendments included that the recipient of an NSL was allowed to explicitly inform their attorney about the request" [1]
Scary stuff. It wouldn't surprise me if there's still some things, like implementation details, that you're compelled to keep secret from your attorney.
Whether or not that would stand up in court is another issue.
Edit: It's not entirely clear to me whether or not this was an actual prohibition on consulting with an attorney, or whether the NSL is worded sufficiently vaguely to make most people believe that's the case. Some case notes from Doe/ACLU v. Ashcroft make me believe it's the latter:
"Because neither the statute, nor an NSL, nor the FBI agents dealing with the recipient say as much, all but the most mettlesome and undaunted NSL recipients would consider themselves effectively barred from consulting an attorney or anyone else who might advise them otherwise, as well as bound to absolute silence about the very existence of the NSL." [2]
"On March 9, 2006 the USA PATRIOT Improvement and Reauthorization Act was signed into law... Other amendments included that the recipient of an NSL was allowed to explicitly inform their attorney about the request" [1]
Scary stuff. It wouldn't surprise me if there's still some things, like implementation details, that you're compelled to keep secret from your attorney.
Whether or not that would stand up in court is another issue.
Edit: It's not entirely clear to me whether or not this was an actual prohibition on consulting with an attorney, or whether the NSL is worded sufficiently vaguely to make most people believe that's the case. Some case notes from Doe/ACLU v. Ashcroft make me believe it's the latter:
"Because neither the statute, nor an NSL, nor the FBI agents dealing with the recipient say as much, all but the most mettlesome and undaunted NSL recipients would consider themselves effectively barred from consulting an attorney or anyone else who might advise them otherwise, as well as bound to absolute silence about the very existence of the NSL." [2]
[1]: http://en.wikipedia.org/wiki/National_security_letter#Histor...
[2]: http://counsel.cua.edu/fedlaw/Patriot.cfm