I have no personal knowledge of this company; I've only looked over what I found on the web. Overall, my judgement is that Pension Benefit Information, Inc. of San Rafael, CA is likely legitimate and aboutmyletter.com is one of two sites run by them (the other being pbinfo.com).
These two sites are registered to Pension Benefit Information, Inc. (aboutmyletter uses Network Solutions privacy service but gives the company name; pbinfo uses their name and San Rafael address.) They are in the BBB. The president (of the 8 employee Co.), Susan McDonald, has testified (PDF on .gov site) before Congress about business uses of SSNs. They made a (very schlocky) video, which has an interview with McDonald after several canned, generic, "impressive" introductions. I found the interview convincing of a person actually running a small, real business of this type. A short version is on their site, long version here. There are some queries about their legitimacy online (like this one), but I found nothing negative on them, and one somewhat positive. One article talks about the suspicions they run into when contacting participants, and has some advice. Also, scammers are unlikely to pay the U.S. Postal Service money to send paper letters.
So what are the dangers? Money or identity. So don't pay them any fees (now or later), especially since it looks like their clients (retirement funds) pay on the other side. As for identity information: What's in the letter? Don't they show that they already know a bunch about you? Old employer? Maybe the last four digits of your SSN? Your address (if this is not the forwarded-by-IRS type of contact letter). Other things, maybe? What information would you be giving up if you did respond to them fully?
You could try contacting your old company directly (mentioning PBI, Inc,), although on their website PBI says you'll have to go through them. (They probably get paid for each successful contact, and deserve it.) Still, responding through mail or telephone to PBI seems like the reasonable thing to do.