Running an ultra at beginning of IM build?

I did a 28 mile trail race about 5 months from my IM this coming November.  It was tough and I had to let the running slide for a week after the race, but I swam and biked so it was OK.  I don't know if it caused a big jump in my fitness, but I did get to practice pain management and focusing on continuing to run when I was toast.  

I think if you can recover quick enough it should be fine.

Is your ultra a trail run or road run? 

I did a 50-mile trail run April 11 (my first race longer than 15K) and IMCdA June 21 (my first, 10 weeks later).  I spent some extra time Jan-Mar running, and the pace for trail running is considerably slower than my road pace, but it was a great opportunity for me to get out and move forward for 11-12 hours and see what I could handle food-wise for a long, l-o-n-g, run. 

I took a week off running and did a couple short rides and a fair amount of swimming.  For a couple of weeks, I could tell my legs were fatigued, but I still got in some decent long rides and was back with a 2-hour long run before the end of the month. 

My biggest concern leading up to the ultra was to not trash my feet.  I couldn't risk blisters or raw spots screwing up the rest of the training.  Second-biggest was fatigue leading to injury.  I was fully prepared to bail after 30 or 40 miles if the conditions were such that I risked hurting myself in a fall or because my gait changed.  Luckily, none of that happened. 

Last year I started ultrarunning and did a 50K in April, a 50 miler in May, and a 100K in June (all trail runs), and then did IMWI in September.  Now, I'm not exactly the poster child for strictly following regimented training plans, at least I haven't been over the past several years.  I've kind of dropped goals like new PR's and such, and replaced them with goals to experience new challenges.  So, in that regard, last year was a success.  I did have some Achilles pain issues that, surprisingly, hampered my long rides more than my running.  Never really did figure that one out.

In my case, with fairly minimal run volume between my last ultra in June and my IM in September, I was still able to run a 3:49 at IM, and exceeded my overall expectations by about 20 minutes.  The big running base seemed to carry through pretty well.

Like others have said, particularly if your ultra is a trail race, and you do a good chunk of your long training runs on trails, that should help you avoid too much wear and tear on your body.

It's all about what's most important to you.  Your plan might not be the ideal way to achieve your IM personal best, yet it might be more rewarding in the end anyway.  :-)