Google tells me it's called a 'reverse rollover' (thanks Logan, I found the same link minutes ago!). And the good news is that I am on the committee that is going to choose the investment options that will be offered in the 401k - so lots of low cost index funds will be there, Fidelity and Vanguard both. :)
There will be no "management fee" on the 401k plan - only the expense ratios of the funds we choose to include. We DO have an annual administrative fee that covers required annual filings, and makes Fidelity the trustee on the plan. Right now they're quoting us $2500/yr with $1.5m assets (or, $500 per partner, which in my case is 0.16% of assets). And as I said that fee goes down the more assets we have in the plan, all the way down to $0 once we get to $2.7m total assets.
As a side note, this is a huge win for me, as DH and I made a kind of boneheaded move last year to move our retirement funds to Merrill Lynch, which is charging us 1.25% (UGH).