I have thought about this method as well.
I am currently on a crash course to clear my Revolving debt and pulled a loan from my 401K to do so. I looked at it this way (from a debt consolidation standpoint) I was paying $200/mo in payments on CC's. A large portion of that was going to interest on the 2 cards that I paid off. My 401K payment is $97/mo and I pay ~5% interest on the loan. I was using after tax dollars before to pay on the CC's and not making much headway due to the interest. With the 401K loan, that 5% interest I am paying, I am paying to myself. It all goes back into my 401K account in the end. On average my account was gaining 3% per year, though I have taken a blood bath through the first part of the year. So, I feel that I will come out way ahead in the end. It cleared my CC debt, freed my credit report of those balances, and I will have the 401K loan paid back to myself within 12 months. Win Win in my book.
Now, using is as an investment vessel could be a whole different story, but I still see it as being a potential tool, especially to help someone that doesn't have the liquid assets to jump right in the game. Pull the $$ from your 401K that you need for the flip. Rehab and turn the property and pay yourself back. I can only see 3 potential risk factors in this method.
1. You lose your job and the balance comes due within 90 days ore you are penalized. Do you feel you have job security?
2. You lose money on the flip, or it takes a while to get it sold, thus draining your resources. How confident are you in the market you plan to buy in? How confident are you in your assessment of the cost of the flip?
3. I don't see this as being a huge detriment, especially in the current state of the stock market. You pull the $$$ out of your 401K now and the cost per share is low. The market takes an upturn before you are able to repay the loan back so the money is being put back at a higher cost per share, thus shrinking your overall position in your investments. This is where you have to factor in your ROI for the project and will the potential long term effects, hopefully positive, from the flip out weigh the potential long term effects of a reduced position in your investments? The flip side of this is that you sell shares for the loan and the price per share continues to drop and you are able to get back in and increase your overall position in those investments.
Ultimately, you are the only one that can make the informed decision as to the best method for your current situation! All we can do is help provide the possible pros and cons to help you see the overall big picture and potential impacts, whether positive or negative!