Hardcopy material isn't the only source of official 4th Edition adventures. Wizards of the Coast provide Dungeon Magazine for subscribers to their Insider service, and by and large this is an excellent publication. Each issue has a handful of fully-furnished scenarios clocking at upwards of 40 pages, and their average quality is significantly higher than anything I've yet seen Wizards offer via retail.
Dungeon #155 offers a collection of additional encounters that can be plugged into Keep on the Shadowfell to, supposedly, expand and enrich the adventure. However, rather than living up to the usual Dungeon standard, these "side treks" are just as uninspired as the adventure they supplement.
The first offering is the Wagon Ambush. This is designed for a level 1-2 party and takes place shortly after the defeat of Irontooth. It sees the remants of Irontooth's kobolds launching an attack on a merchant caravan headed to Winterhaven.
It's as redundant as it sounds. This is yet another kobold encounter in a story overflowing with kobolds. It's also the third kobold ambush in the first six encounters. If players weren't already thinking of 4th Edition as "the kobold ambush boardgame" they will be now.
More importantly, it's anticlimactic. The virtue of the Irontooth encounter is that it finishes off the kobolds and paves the way to the keep. Going back to kobolds takes away the sense of progress derived from that very difficult fight.
The encounter also includes some new types of kobolds: a kobold hurler, kobold pikeman and kobold slyblade. The kobold hurler is just a level 2 kobold slinger; it's the same monster, complete with annoying glue pots, just levelled up. I'm not sure how I feel about this kind of thing - it's nice to be able to run the same monster at different challenge ratings, but a one-level difference barely seems to justify it having a new name.
The slyblade, by contrast, is genuinely interesting, a level 4 monster with the ability to deflect attacks onto allied minions and initiate a dangerous "twin slash". The pikeman, too, is pretty nifty, and fits well into Keep on the Shadowfell by introducing the concept of "reach" - using his pike, the pikeman can make melee attacks against enemies up to a square away.
The encounter concludes with the players finding a trail of bootprints leading from the ambush and heading towards Shadowfell Keep. This could be useful as a prompt to get players back on track if they've missed all the signs pointing them to the main adventure. It's not explained who made the bootprints, though. Ninaran? Some goblins or hobgoblins? We may never know.
 If the players don't defeat Irontooth, and instead proceed directly to the keep without clearing out the kobolds, this encounter would be an excellent way to underscore the consequences of their decision, while providing some of the catch-up XP they'll need to compsensate for skipping the kobold lair.